Since Atlas is not taking submissions:
Secrets of Hermes: Four Chapters: Secrets of the House, Secrets of the Tribunals, Secrets of the Houses and Secrets of the Magi.
Familiars
Hermetic Libraries: A book about Hermetic books. Reprint the large library and political pamphlet rules from TME, revisit book scores, add information about mundane tomes and libraries and include a very large selection of new spells. Also rules for the printing press.
Tribunals of Hermes: Rome
Tribunals of Hermes: Iberia
House Diedne
The Silk Road
Mythic India
The Golden Horde
The Crusades
Ars Magica 6th Edition (yeah I've been saying that for years).
Adventure in Mythic Europe - a set of adventures that are more Indian Jones and the Temple of Doom and less explore some aspect of the middle ages.
Hermetic Crimes (stories that revolve around breaking the code).
Hermetic Diplomacy (stories that revolve around the orders interaction with other groups in ME).
Cities of Mythic Europe (more info on major cities).
Shamans: Revised Edition. - Everything you ever wanted to know about spirits in Ars.
Manor & Fief: Dealing with the peasantry. Also including floor plans for manor houses, towers and castles
Long Sagas: A book on taking the game through at least 1492.
Subsystems: A short web supplement listing various game styles and what books you should use.
The Hercynian Saga: A complete saga-in-a-book detailing the unification of the Hercynian Forest Spirit of Germany.
The End is Nigh: A complete saga-in-a-book combining the rise of the Titans/Muspelli, the Antichrist, angels with Trumpets, bitter Olympian gods releasing the Kraken, and all manner of other apocalyptic tales into one high-octane, high-fantasy, brief and explosive action adventure saga for elder magi.
A Byzantine Fantasy: An alternate setting book. The last vestige of the Roman empire is beset by Orcs from the east, Centaurs from the north, and the demons and sorcerers of the fallen West. In a desparate move to save his Empire, the Emperor has established a new Order - the Imperial Order of Hermes, an order of wizards sworn to defend the Empire. You are one of the Founders of this new Order - your actions will determine the fate of the Empire, and can shape the place of magi in society.
Ars Magica, Open Gaming Edition: A port of Ars Magica's to D&D/OGL games. Based on advancing Arts with Level and modifying the spell guidelines to conform to d20/OGL/Pathfinder guidelines on damage and special abilities.
I wouldn't care for a 6th Edition, but I certainly wouldn't mind a 5.5 edition (or even just a Storyteller's Companion (as it were))- ie, one that contains all of the rules clarifications, and points out where the issues that a Troupe needs to decide on themselves.
Changing mythic europe has a magical printing press that basically outlines the idea.
The Fallen Gods
something that covers Fredrick II's war with the Pope.
My own sugestions
(covonents) oH
Book detailing 3-5 different covenants. To be used either as an NPC covonant or a Saga starter.
(Unearthed Arcana) oH
Difficult to describe. Basically a book of Design notes, clarifications and optional rules along the lines of the SG's guide or Subsystems books mentioned before.
(History) oH
Exploration of how the order has changed over the years. A Resource for sagas set in other periods as well expanded background for the default starting point.
(Renegades) oH Hermes
A collection of Marched Magi and assorted outlaw Npc's along with setting information on wizard marches and operating outside the Order of Hermes.
Roman Tribunal Book 5th
Of all the Tribunal Books from earlier editions Rome just doesn't fit at all anymore.
I'd like to see another Antagonists book. I thought that was well done, plenty of adaptable story material and compelling antagonists. More of those, perhaps of some of the Rival Magic traditions or Hedge wizards.
This likely won't be well received, but perhaps a saga of a couple of lengthy adventures with far reaching consequences and long period of time (maybe a century or more).
Some of the previously published Tribunals need an update to the current edition as well as the current overal story line. As was mentioned, Rome and more than a few others are just out of place at this point.
An epic adventure such as the Titans vs the Gods, like someone mentioned would also be cool.
I think Demons and Devils need some more meat. I liked the material printed, but to be honest the demons didn't seem very threatening due to their shortcomings in how they cannot have any virtues. So some more demons that are perhaps more powerful as well as clever.
I'd like to see a Peripheral Code book, perhaps in conjunction with a book on "Tribunals" - the political event, not the geographic boundaries. How they're run, plothooks, a calendar for the different Tribunals' Tribunals, maybe a collection of extant rulings - all in one.
The PC part would have to be carefully written, perhaps even "playtested" somehow to avoid obvious (unwanted) paradoxes or undesired implications, but I think a lot of the more "extreme" confusion of the PC and The Code itself can be resolved more easily. (Lots of room for "Tribunal A rules yes, Tribunal B rules no" for areas that "should" stay grey.)
History of the Order
- Things that led up to the foundation of the Order,
- Important things that has had a great impact of the Order
- The Founders
- The Founding of the Order and the first Tribunal
- The most important Covenants
Secrets of the Order
- The Archmagi
- House Diedne
- The War against Damhan Alleid (spelling ? - I do not have my book here at work)
- Secret cults within the Order
- Enemies of the Order
Rome Tribunal
- A usual tribunal book
Iberian Tribunal
- A usual tribunal book
Loch Leaglean Tribunal
- A usual tribunal book
Tribunal Politics
- Descriptions of the tribunals
- Description of the Grand Tribunal
- Plots and hooks during Tribunals
- Various periphial Codes
Beyond the Borders: The Silk Road
- Mythic Asia beyond Samarkand and what is mentioned in the Crescent and the Cradle
Beyond the Borders: The Ring of Fire
- Mythic Africa
Beyond the Borders: Heaven of the Holy Heffers 
(Yes we need a better name for this)
- Mythic India
For the one on India, I'd personally expect all sorts of mythic, mystical stuff....
and then Ghurkas. Not even mythic ones, the real ones are scary enough. Quote of the day: Fortune favours the Ghurka. Or else the Ghurka hunts it down and kills it.
5th edition versions of Mythic Places (with focus on potential stories and adventures) and a Wizards Grimoire or two (a junk drawer of cool magic gimmicks and spells that are pretty simple and plug/play worthy)
Like some of the others I'd like to see the 3rd Edition Tribunal books redone. There are many good suggestions mentioned previously that I would buy. I'll try to post something different below:
1.) Ruins of Mythic Europe - a book detailing various ruins in Mythic Europe that have not yet been found or fully investigated and plundered. Details loot, Ghosts and other Monsters/Creatures/People in the ruins. *Note: I have a player who wants to be a ghostly historian - he interviews ghosts to get ancient area lores so something like this would be super useful.
2.) An apprentice adventure book - "Tales of the Apprentices" could be interesting.
3.) The true history of the Order of Hermes. The history we've been told in the core rule book is for the most part a lie. This is what really happened.
4.)
One idea I did have is something in the vein of Hermetic Projects, but instead of focusing on large-scale projects it would instead have a bunch of spells and enchantments designed to perform tasks that are normally difficult to accomplish via vanilla Hermetic Magic, expounding differences between Muto/Rego with regards to Imaginem/Mentem spells and similar minutiae of Magic Theory.
I would like all those please, plus the real-world spell that creates extra free time and halts aging.
... an interpretation of Ars Magica for contemporary urban fantasy.
Well, I can't comment on some of these but for the topics below, my blog will be covering some of these in some depth over the coming months to years, basing mainly off cutfiles and undeveloped ideas from my various official projects, which may be about as close as you'll get to official material.
Familiars (including an expanded bestiary) and remakes of all the pre-5th edition tribunal books (most importantly Rome).
I think your best bet is to adapt the rules from Mage: the Ascension or Mage: the Awakening. Awakening is still in print, and Ascension just went through a 20th edition anniversy kickstarter.
Thanks! I'll try. In the meantime I'm trying to cobble together a home brew rpg oriented on the idea but am figuring on pulling in as much of ArM5.0 to hopefully elide into Mythic Europe proper one day. My teen sons and maybe a friend as troupe.