I would agree that 3rd edition material needs most work to be integrated to 5th edition saga.
Amongst them Iberia would interest me the most then Rome.
Our IRL group played a saga in Loch Leglean a few years ago, I agree with Here Kane that It needs less work than the others to adapt. The two others are basically off limit when we select saga location together.
There's hope for the 5th edition line, that's a nice Christmas gift.
Loch Leglean is a WotC product, compared to Iberia and Rome which are WW products. A lot of the problems with the WW products is how they were trying to fit AM into their WoD, especially their Dark Ages material. Dark, decaying, and controlled by ancient evil.
I would champion the Levant (although less deserving of a new edition then the third edition books) since there is a lot of potential in such a tribunal, at least in my opinion. Conflict with the Suhar Sulayman is very enticing for me as a GM (and someone hungry for Suhar Sulayman stories).
All of that being said, I am mostly unfamiliar with previous editions as I got into Ars magica back in 2018.
All in all, I very much care that Ars Magica gets official support into it's third decade of fifth edition.
The Levant is certainly a very eventful setting for a saga (you get two crusades in the first 10 years if starting in 1220...).
The content of the Levant book is still largely compatible with 5e I've found. It mainly suffers from the fact that it spends more than 1/3rd of the book introducing Islam, Sahirs, and Jinn leaving less space for the description of the tribunal itself. This is similar to the Loch Leglean book where a good chunk of it is taken up by character templates and the rules for Grugachan leaving it a little thin on detail in other regards (in comparison to other tribunal books).
I'll also note, if you're waiting for new books, there are 22 issues of Sub Rosa, which you can rummage through... PM me if you want details, or search the forum for one of my many posts.
EDIT to note: We make no profit on Sub Rosa; it all gets rolled back into making more Sub Rosa.
Which reminds me: any news on the next issue?
I care.
And now I'm going to sit on my hands and not interfere further. 
I think you can find good reasons to revise any of the books I think.
My vote is for Rome. A WW product like Iberia, it offers very little that I find useful, not only because it is in line with the 5ed world view, but as much because the style of the time emphasised individual antagonists with stats, and offered very little on the everyday customs and relations. Furthermore, Rome is an important tribunal which cannot be ignored, with three domus magna and the cradle of the Mercurian tradition. Rome is one of the tribunals characters are most likely to visit or correspond with, regardless of where they live.
That said, I think the entire line has to be rewritten as a single, living, hyperlinked omnibus. The inconsistencies within 5ed bother me at least as much as the inconsistencies compared to 4ed. The various magical traditions are handled partly in tribunal books, partly in tradition books, partly in mechanical rulebooks, and partly in house books. It is a complete mess. Ars Magica tries to be something which does not fit into standalone books.
If you stop and consider the sheer amount of traditions described in 5e you're talking about probably something like 500-1000 pages of content over the whole product line. I don't realistically see how that much content could be put into a single omnibus, or that doing so would end up profitable when you consider the effort it would take.
Maybe you could have an SRD style reference document providing just the mechanics of each tradition (and maybe of other things), minus the background and flavour. But that also comes back to cost-benefit I think - would that document actually make money, or attract enough new players (who then purchase books) to be worth the resources spent making it? I'm not sure.
In practice the 5e content that gets regular use isn't that scattered either imo. Depending on where a saga is set you probably have the core book, the three Houses of Hermes books, and Hedge Magic and/or Rival Magic plus a tribunal or setting book. In my experience anything beyond that only gets used by specific players or for single adventures here and there rather than regularly referenced, but maybe for other people it's different?
5-6 books is quite a few, but it's not disproportionate to other rules-heavy systems. Look at the 40k rpgs that FFG published - 5 game lines for one setting, each with at least a half dozen books, and they're very popular. Or Pathfinder, which had about 30 books not including adventures and setting books (that's 1e, before it had an SRD).
The size of the content is no issue. Wikipedia has much more than that.
I whole-heartedly agree on profitability though. I might, just might, work as fandom, but as commercial publishing, I can't see it working either.
I would LOVE to see a 5th ed treatment of all the remaining 3E tribunals. I'd say Rome could even use two supplements, there is so much material I'd love to see covered.
A book outlining a Tribunal or a Grand Tribunal would be really useful. I can't remember when the next GT is in cannon, but an outline, some characters and lots of story seeds would be really useful.
I believe Guardians of the Forest has some guidance when they describe Durenmar, which hosts the Grand Tribunal.
I would very much love to see all tribunals done under 5th, especially Rome.
Start with the 3rd, they're the most needed, but even an update for the 4th would be great.
It is a hugely important tribunal on all fronts, and what we've got isn't great.
After that, give me anything ars-related and I'm game 
Taking up my Party-Planning Scenario Structure and scaling it up to do a Tribunal is definitely a thing I want to do!
I second Ben's recommendation for Sub Rosa as a great source for ArM material.
Sub Rosa #16 865 AD Voventes Centennales contains two pages detailing the 6th Grand Tribunal of 865 - from a different time in the history of the Order.
The Italian folklore I'm reading right now has Sardinia as the home of the powerful faeries. It also had a reference to a pagan euthanasia ritual I need to follow up.
Thanks for the link. I have started preparing for a tribunal and this will come in handy.
These are the books I think would mostly benefit from a re-write, in rough order of priority:
Those two would be the ones most in need of a 5th edition remake, IMHO. I'm currently starting a saga in Rome, and had to throw mostly the whole Tribunal book out. There's a couple concepts you can keep, but they're really tiny.
As for the Iberian one, I have less experience with it (I'm from there, so I prefer more exotic locales), but I've read it and I also feel there's little to be salvaged there.
This one, though outdated, is arguably more usable than the WW ones. It was a later book, and that shows. There's stuff that's not longer relevant or correct, but at leas the tone is kinda right, and the percentage of useful material is high.
These are the books I don't think really need a re-write, though of course would benefit from one, also in priority order:
I extensively used this one year ago, before 5th edition was around. Though it was a long time ago, and my memory fades, from what I recall I think it would need little reworking for 5th edition, besides the problem with Blackthorn (addressed elsewhere). So I don't think this is a high priority re-write.
Can't really talk about this one, have never used it. Even though I do own it, I haven't actually read it, but based on how close 4th edition material usually is, I'd think it's probale highly usable.
I have extensively used both these books, as our current saga is centered in the Theban Tribunal, with close ties to the Suhar Suleyman and some of the Novgorod covenants. I find them perfectly usable, and found no need to throw anything away. In my personal opinion, they're fully usable as-is.
And finally...
I do care. I do strongly care. I've been dying to see a new 5th edition product, and in particular a 5th edition version of the older Tribunals would be awesome.
Also, I'd love to see what you do with Ars Magica, Justin, since I'm a long time reader of The Alexandrian.
Edited to add:
On an unrelated note...
I've actually done this, and can attest to it working like a charm.
I guess you must be talking about the "accabadora" practices. If that's the case, you could be interested in watching an interview with an old sardinian woman about her experience with one of these figures. The interview is in mixed italian/sardinian language, with italian subtitles. It should be not too difficult to follow.
These practices have a strong "folk magic" vibes, i always found them really fascinating.
And for the other part you mention, about the faeries of Sardinia, you can find some really interesting stuff. The "Domus De Janas", and the nuragic holy wells.
Some links
