Not every Tribunal got a 5th Edition supplement. These include:
3RD EDITION
Loch Leglean
Iberia
Rome
4TH EDITION
Levant
Novgorod
Stonehenge
Greater Alpine
Which Tribunals would most benefit from a 5th Edition update? How much material needs to be reworked? What new material would be most beneficial for each Tribunal?
Do you even care? Or is what we have good enough?
Novgorod would be excellent, as well-described covenants and areas of this wild frontier of Mythic Europe would really inspire players, as could good use of Russian mythology.
Rome and Iberia could benefit from not having demons everywhere and using more history.
As a newcomer to the game, any Tribunal not covered in 5e would be great, especially if it last got a book in 3e.
I enjoy very much the in depth look at regions, and the story hooks they bring with them. Sure, some crunch is nice, but it's mostly the fluff. Who are the movers and shakers, some details on existing covenants, and some of their Magoi.
I would say the 3rd edition books are both the most needed and the most work to rewrite. 3rd was the time dominated by WW where the game took a much darker decaying and demon dominated turn, so those books have a lot in them that becomes mostly wasted space when trying to glean things usable in 5th edition from them.
The 4th edition books (especially Stonehenge with its complete lack of rules) are much easier to glean useful 5th edition material from for a SG.
From a personal desire, I would say the Greater Alpine followed by Loch Leglean and Rome, with Novgorod in the fourth spot. Those are just the areas I am most interested in, but they would give a good coverage of variations within the order. Greater Alpine and Rome has the oldest most established feel for the Order, though they are on complete opposite sides from an isolationist standpoint. Loch Leglean is on the edge of the Order but still enough within it to not make it feel totally isolated. Plus I really dig their hedge magic and big bad. Novgorod is great for players who want to be pushing outside the tradition area of the Order and experience things that are more exotic.
The 3rd edition books for Rome and Iberia do no longer fit with 5th edition Mythic Europe and the Order of Hermes: IMO they truely require a complete rewrite.
The book for the Levant (2002) is partly superseded by The Cradle and the Crescent (2011): so a new book about this Tribunal could be slimmer, or treat additional topics - perhaps far trade routes and their destinations. A rewrite of the book about Loch Leglean might also be rather slim, as it can rely strongly on Hedge Magic Revised Edition.(2008) for magical traditions.
The covenants from the (2003) Greater Alps book fit well with ArM5 and need no rewrite.
The books for Stonehenge (1999) and Novgorod (1999) were published close enough to ArM5 to be still very useful with it.
EDIT: A Loch Leglean rewrite might become more substantial, if we add the sea voyages and colonies of the Norse to it. It could thus replace the 4th edition Sanctuary of Ice (2003) as well.
This is very high up there on my Ars wishlist.
The Rome tribunal book is the #1 here by a country mile IMO. It's one of the oldest tribunals, it contains three Domus Magnae, it's the centre of the redcaps, the quaesitors, and the church, historically was presumably the centre of the cult of Mercury, the native country of Bonisagus, the place where Tytalus, Tremere, and Pralix were initially trained... It's a significant tribunal for the order, to say the least. Sadly the 3rd edition book is pretty incompatible with 5e, not only mechanically (to be expected) but in terms of the ideas not matching with other editions.
Loch Leglean and Iberia would be tied for #2. They have the same issues to varying degrees due to their age.
The 4th edition books are much better. I prefer the extra detail of 5e tribunal books, but the 4e ones are still perfectly usable. The biggest improvement in the 5e books, and something I sorely miss running games set in tribunals without a 5e book, is the lack of focus on how the tribunal functions in a hermetic sense - knowing the little idiosyncrasies, the hermetic culture, and a bit about the peripheral code of a tribunal is pretty important to actually playing magi in it, but the 4e books largely leave this open ended.
For example, The Lion and the Lily (5e) has 20 pages dedicated to hermetic culture and history, while Heirs to Merlin (4e) has about 6 give or take. Those 20 pages are the part where what makes the tribunal unique (from a hermetic perspective) is established and explained, so it's quite crucial. The game is about the Order of Hermes after all!
Rome and Iberia are the key ones that really need rewriting for me - in particular, it should be possible to pick up a rock without thereby furthering an infernal plot.
How detailed do you want suggestions for what we'd like to see be? I have some thoughts on the broad strokes of how I would update them for 5e (particularly Rome), but not sure if that's crossing a line.
Loch Leagan I don't have a very strong opinion on - the basic idea of a Tribunal on the outskirts of the Order full of hedgies works well enough, but there's the question of how to make it distinct from Hibernia and Novgorod (although Hibernia's quite distinctive itself). The current book doesn't really grab me.
Stonehenge could do with a bit of tweaking to make Blackthorn fit with 5e Tremere, but is basically fine.
The Greater Alps is also basically fine. It might be useful to see more on Archmages and other very senior mages, and what they do with their time.
Novgorod I again don't have any strong opinions on. Ditto Levant.
I believe those from 3rd edition would benefit most. Of those, personally I'd go in this order:
This is all great insight.
I've also been thinking about whether there are any features the existing Tribunal books don't have -- or which perhaps only some of them have -- that people think would be useful to have?
For example, I've been looking at @Timothy_Ferguson's Cornwall, with its packaging of locations appropriately seeded with content for player-created covenants and player briefing sheets.
While I do like the idea of a list of researched locations with pre-seeded content for players to use when creating their covenant, what would have to be cut to achieve it?
Because what we are talking about here is not a fan made product, in which word count and research are limited only by how much someone is willing to give. For an actual product, in which writers, researchers, and typeset/layout/art has to be paid, then word count and research cost. If there is going to be a physical product (rather than just a pdf) they cost more. All of the current tribunal & world books clock in at around 144 pages, except one. Only The Cradle & The Crescent breaks that size, along with having an increased price.
So while I do like the idea of packaged covenant locations, it is not something I want more than any sections in the current format or see as something justifying any increased cost book.
However if Atlas is thinking about/planning more than a few new Tribunal books, "Covenant Locations" might make a good Scenario Book. Or possibly even a digital only series for many of the current Tribunals if you are feeling especially froggy. That is something other publishers have done successfully and has the possibility of taking advantage of your prior work and research, especially material that was cut.
(Side note: Dies Irae on your scenarios webpage uses the Antagonist cover.)
If they are published, I will buy them. I'd prefer Iberia, Rome and Novgorod before any others.
Something that I always thought would be a good addition would be a small spotlight on the "movers and shakers" of a tribunal - a slightly more in depth look at the key magi of the tribunal and their motivations (from a hermetic perspective), their allies, enemies, etc. A sort of "Tribunal Politics" section.
That content is in the 5e tribunal books to varying degrees depending on the book, but it's scattered around the entire book generally. From a practical standpoint of running a game it would be nice to have a "here's the factions and the big players in the tribunal as of 1220" summary. For some tribunals that would just be a summary of existing material, for others it would have to expand on the relationships and motivations of NPC magi a bit.
I've mentioned it elsewhere, but I would prefer if the adventure seeds were kept separate from the descriptions in the book. Like having a separate chapter at the end of the book which is aimed specifically at the ST which would concentrate them. I am running the Rhine Gorge saga, but I specifically asked my players not to read the Rhine book because I use some of the prewritten seeds and I feel that it ruins some of the mystery.
Edit: as I think more about it, having a big book of seeds, which could be transposed each in 3-4 Tribunals would be great. For example Calebais has a few preset locations for some tribunals.
The main reason that I have been putting up some of my adventures is that as I started ST-ing, I found that there was a lack of such "modules", something that's easy to grab and run.
So a book with a mix of general seeds, like a few chapters sorted by who the source of the problem is, each a full page. A chapter of token monster hunts, a few church problems, problems with nobles etc Just a clear intro/background at the top of each page, one column with more details on the actors/motivations/stakes, and one column on consequences and rewards.
I’ll agree with what many others have said here: Iberia and Rome are at the top of the list, simply because they interact with the rest of the Order so much. I’ve been working on an Iberia rewrite for a while, and you’d be surprised how often Iberia is referenced in 5e products. For example, did you know there’s a large single-house Tremere covenant in Iberia? Or that young magi recently raided Duresca hunting for the scrolls?
Stonehenge is built around the idea of Blackthorne as Bad Guy whose existence unifies the rest of the Tribunal. This no longer works in 5e. I like the book, but it’s a table built on three legs.
As to things missing from some of the books, Loch Leglean has no map, a tragic flaw I’ve griped about elsewhere. Other early maps (Iberia) are really bad. Rome, Iberia, and Loch Leglean describe 4 covenants each, while newer books discuss almost all the covenants in the tribunal, which is much more helpful to me. Recent books have been great at spotlighting potential covenant sites—more of this please! Along with vis sites. Maps need to have legendary or magical sites on them. They don’t even need to detail those locations—a sentence of body text is enough—because I can research them, but they need to show me the site exists and give me someplace to start my research!
I was so focused on answering in my prior post, I forgot to say the most exciting thing in this thread is that it was started by Justin Alexander rather than just another of us fans!!! So there is at least some interest at Atlas Games. Yay! 
Ivery much agree with the "more potential covenant sites" sentiment. I went through the Rhone book using those up to pad out Fenghelds chapter house list.
Rome and Iberia are probably the ones which need the biggest rewrite and I agree it would be helpful to keep as much secret and story seed information as possible in its own chapters or for something not being printed as seperate document , so that people not planning to be an SG can read freely without stumbling on a potential spoiler
I've been running a 5e game in Stonehenge and I've found that Blackthorn as tribunal antagonists still work, so long as you give them a decent motivation for trying to control the tribunal.
We just tweaked it so that they genuinely believe that Stonehenge is a "military border" of sorts for the order. Between a suspicion that Davanallus might not be completely dead, all the Diedne sites in the British Isles, the fear of Rune Wizards from the north, and the powerful hedge wizards of Hibernia and Loch Leglean, there are enough boogeymen that a "we must trade freedom for security" stance is sort of understandable.
Everything else is exactly as it is in Heirs to Merlin and it works quite well. They are still firmly antagonists to most of the tribunal, but their actions have a purpose beyond power for the sake of power. From the perspective of other covenants they are power hungry and dictatorial. From their perspective everyone else is putting their personal desires above the safety of the order.
Not that I wouldn't buy a new Stonehenge book in a heartbeat, but my experience using HtM with 5e has been that it needs very little tweaking to work.
I've taken a similar but slightly different tack in Stonehenge, in so much as I had tension between Blackthorn and the local Mercere due to Blackthorn arguably overreaching by trying to ban the training of Bloodcaps following a number of grisly murders. And then a Gifted Bloodcap child appearing at about the worst possible time in the debate.
So you've got Blackthorn being heavy handed but you can sort of see why someone saying "actually, training people with a lineage that inclines them towards violent murders is a bad plan" might mean well by it. You've also got the factor that the Mercere are a significant source of news, so members of Blackthorn were finding it very hard to build up a positive reputation in the Tribunal, and very easy to build up a negative one, which along with the fact that Blackthorn had been dominating the tribunal for a long time meant that a significant proportion of the tribunal didn't have a high opinion of Blackthorn. So Blackthorn had a good incentive to work directly with the PCs if they could.
I think I did later find something in canon that it didn't quite gel with - might have been a statement that Dhampirs were allowed as ungifted Redcaps in Translyvania, or implying that Tremere and Mercere usually get on well. I don't think it was a huge contradiction, though.
For Rome, I tend to head-canon it as having many of its covenants engaged in siphoning off vis from the rest of the Order (due to its scarcity there), often in competition with each other (it's 5e canon that it's fractious). So you've got covenants selling magic items, (Mercurian) ritual casting for hire, engaging in the book trades, mercenaries..Lots of trying to do-down the magi vying for the same bit of trade, with Magvillus and a large Tremere House Covenant that's getting fed vis from Translyvania trying to act as stabilising influences.
One thing that would be interesting to me in a new book is more about how the Hermetic services market (and Hermetic underground) work - there's a bit on the Mercere specific bits in HoH:TL, a bit on the book trade in Covenants, and info on Verditius prices in HoH:MC, but there's a lot of room for expansion still. Statted NPC magi who you can commission things from would be nice to have, at least for me (I suspect SG's might occasionally run into the problem of players arguing that they can absolutely have this thing because there's this NPC who will sell it).
That’s super interesting and I’m gonna steal it, thank you!