So most of us know what's up the pipeline, but after those books are finished what then? Also we can dream that David and John will take some of our suggestions into concideration, so lets have those ideas!:
My suggestions:
Quill and Blade: The Mythic Europe Guide to Companions. Things like advanced combat rules, new professions, templates, V/F's.
A sort of fringes of Mythic Europe book where you deal with rumors of the lands beyond Mythic Europe, like: The thousand wonders of India, Persia, the lands "South of the Sun", the lands beyond the Silk Path, even maybe Hy-Brazil or Atlantis.
HOT
Well, I know this idea wouldn't be received all that well, but I've always hoped for a Ars Fantasy companion book. Good rules for stepping away from a purely Mythic Europe setting.
I have House Rules I wrote up for 4th Edition where we replaced the calculated techniques of combat and made a Combat tree similar to the Magic tree. Instead of Creo/Intelligo/Rego/Muto/Perdo the combat tree had Speed/Accuracy/Defense/Power/DualWield and 10 'forms' of combat (such as unarmed, axes, swords, polearms, etc) We even had some Maneuvers similar to spells, but that tended to mess the game up more than help it. It allowed the game to feel a bit more 'fantastic'.
I'd also like a companion to deal with non-magic/non-combat stuff. There's no rules for doing anything other than that. How fast does a person move, how long can they hold their breath, how difficult to walk a narrow ledge, and include a longer list of beasts that is a bit more filled out. It could be fluffed easily with examples and scenarios.
That's my Wish List. 
Sphynx
I love the notion of the Companions book -- didn't we already see some hints that this would come out sooner or later?
Of course there are two books that will come out that I am really looking forward to -- RoP Faerie & Magic. Glee!
And I would be fascinated with a book on General Trends In Mythic Europe -- magi can live many, many decades, even centuries. Time (and society) does not stand still. What innovations would arise? How would magi cope with the alterations to the world around them?
And this would feed straight into Ars Magica a la Jonathon Strange & Mr Norrell...
Yes , a Companions Book would be most excellent!
I would like to see the long awaited (for some)
Herbalism and Alchemy rewrites.
Rapidly completing the Houses of Hermes and Realms of Powers series would be nice.
EDIT: +scenarios (yeah, I know 8)).
The House Books soon please!
I'd settle for the Mysteries to be honest...
Covenants, The Mysteries, Faeries and Magic, and the House books are all books I want to see already, so I don't know if there is anything more pressing.
A slew of adventures (or encounters or so on) would be very nice too.
I'm not too keen on another tribunal book, really.
I would really like an Ars Fantasia book, a cohesive and thorough system to play D&D-style fantasy with ArM mechanics. ArM has lots of things I prefer mechanically, but I've found adaptation will require some serious time and thought.
Inspired by a discussion on the Berkelist, I wouldn't mind a sort of Mythic Europe for Dummies sort of book. Lacking a lot of the historical information I wouldn't mind a Mythic Nobels, a Mythic Church, or the Mythic Crusades in one form or another to add gamable information to my saga.
Concerning which books come out, I have to say I don't actually care so long as they contribute material that is relevant to the Hermetic world. This has not been a problem with Ars5 books, but previous editions of the game often had books that provided interesting material, but often material that had little or no use for wizards.
For instance Mythic Europe is a fun book, giving a massive (if confusing) amount of information on the numerous cities that make up Europe. However there's hardly a paragraph in that book that one could call Mythic. I think somewhere in the back of the book they make one mention of the Tremere and the Order of Hermes.
While it's handy to have a quick appendix of medievel europe, it's an appendix that has almost no influence or bearing on the wizards of the Order of Hermes. As a player of a Hermetic Magus, I want stuff that's relevant to my characters.
So while Mythic Nobles, Church, and Crusades all sound interesting, what I want is something that contributes story material for wizards and grogs alike.
Ordo Nobilis for 4th works ok as a realm book for Ars Magica. If you ignore the mechanical side it has lots of info on the different countries of Europe in the early 13th century. So as a "ME for dummies" it's good enough. Also the clarification on (Area) Lore, is goodness.
how about a book of the holidays & how the mages use them?
Out of curiousity ,
what is wrong with the Mechanics (such as there are any)
in Ordo Nobilis?
Nothing really, just assuming that most of us have moved to 5th ed now. If you play in a 4th ed campaign it has a couple of good virtues, monetary systems and revised combat rules that's good enough.
HOT
Yeah , guess i thought you were speaking in context of 4th Ed.
As i didn't have the 4th Ed books long enough before 5th Ed was published ,
i haven't bothered looking at any system mechanics in them.
The Ars Magica Calendar created by Hermes Portal magazine has special days charted for both Hermetic and non-Hermetic use, for the around 1220 (1223 I think it is). The calendar is also, coincidentally, usable for 2005...
It also has 12 excellent monthly illustrartions and one very fine cover.
It was sent to subscribers to HP13/14 - I'm not entirely sure how you get it now, if you hadn't bought those - I guess you chase Eric directly.
http://perso.wanadoo.fr/styren/hermesportal/hermes1.htm then click on News (where it is listed that the calendar was released) or Contact or "How do I Subscribe?"
I would like to see a supplement that covers the early days of the order especially more info about Diedne and the schism war. Of course I wouldn't use it in more current saga as it would likely conflict since I'm currently running a 1001 AD Vinland saga. Also (if it isn't in a prior source book) info on the Cult of Hermes and other ancient organized magic traditions such as Egyptian.
here's another idea for a book.
a book of mundane/magic items perhaps?
I agree this is an interesting time. I suspect that specific nature of that time would make it hard to justify publishing a book because it wouldn't have any practical use for 'contemporary' games. However I could imagine a book that occurs in 1220ish Mythic Europe and deals with issues based on the history of those times. That way you get a 'current' book and something that deals with the past as you imagine.
On a different note, my brother GM'd a campaign that was set at the foundation of the Order. The players played either students of a founder, or magi who joined the Order. The Aegis of the Hearth had not been invented yet, Certamin wasn't widely known, and Gurnicus was still discussing the situations where Wizard's War would be used.
Into this setting, the characters came as 'regulators' who purpose was to teach magi the 'new' ways of the Order. The campaign had a heavy level of both politics and fighting. It was a refreshing change of pace and I recomend others take a break from normal Ars to explore this ancient time.
maybe a story-line book, or a really big saga...Like those olds campaigns for other games...