A smaller city has the advantage that it can be located anywhere. A city like york is still very important in the time period, but you can transform it into Liege, Fois or Toledo with minimal changes. That helps a lot if you are not going to a capital city regularly.
Several cities would be neat indeed. I would vote venice because it is the hiub of the rome tribunal, paris, london, tolouse, barcelona, toledo, dublin and vienna. Add Accre, Tyre and Alexandria for my wishlist 
Cheers,
Xavi
Funnily enough, many of these would be quite acceptable to the Order.
Mass Mind Control is not only fine (as long as you don't cause problems for your sodalis) - as well as entirely possible.
Same with magical diseaes I believe.
Figuring out how to grant the Gift would likely make you the most famous magus after Bonisagus.
Binding mortals as familiars? Why would this be a problem? There are probably 3 bonisagii looking at it even now!
Permanent destruction of the Gift is the only one that would get you into serious trouble I think
You are forgetting the first (admittedly, unwritten) precept of the Code of Hermes: thou shalt NOT GET CAUGHT! 
Eric
Actually for all of the carry on we have about Venice, I think that Palermo or Naples would be more interesting choices.
No offence ot Shannon, who was writing pre-Google and pre jump to 1220, but we could do so much better if we did a new Roman Tribunal book. I mean, so much better. I mean, it's a book with Palermo and the HRE, Naples and the necromancers of Avernus, Venice and the Hermetic city of spies, Rome and the Pope, Salerno and the new universities, Florence and the rise of the Medici. I'd want to push Verdi out of Italy just so I space ot do other cool stuff in the book, because you'd not have -space- to do all of those Domus Magnae.
There are 3 domus magnae in the Rhine tribunal as well and they do not occupy too much space. I do not see that as a huge problem, really 
Unless you want to go in great detail in each, something I do not specially favor myself. The rest of stuff you mention (important and interesting city's galore!) would be much more useful to any given saga. Naples has somewhat been covered already in Rival Magic and IIRC Verdi is mentioned in some detail in MC.
Xavi
Then make it so! 
I wouldn't mind 5th Edition rewrites of the old Tribunal books.
That's a move I'd not support though.
OUTLAWS
A book about magi whomhave been Marched.
Like the Magi of Hermes book this could be a detailed write up of half a dozen Magi who have been Marched. Their life and crimes.
You could have;
A diabolist on the run yet looking for redemption
A falsely accused wanting justice.
A Mage whomhas been on the run for decades yet never been caught. What is his secret?
Old stories of Founders apprentices who have been Marched.
A Mage on the run from a secret Mystery Cult.
Someone exiled by the head of a House.
A Tytalus arch Mage who has himself exiled just so he can dare anyone to take him on in the ultimate challenge.
Pc's could be prosecutors, hunters, or perhaps a friend of theirs has been Marched. They could hide or otherwise aid them. They could help prove their innocence (or Guilt) or they could join in the chase.
The book could have a general section of how Marching happens, what happens next, and how you could go about being reinstated. There could be a section on how you go about forming a posse and different methods of hunting. And ways of fighting back.
Actually, that's easy to do. The Verditius domus magna has moved to Ingasia, in the Thebes Tribunal. The Contest, the trophies, the pageantry, all went east. Verdi will soon become a Verditius backwater, a ghost of its former glory. You could stick a few cantankerous magi there, sour Verditii that refuse to leave their forges, but the political focus is gone.
Matt Ryan
The "problem" is that verdi might not be the domus magna anymore (even if that is contested in the Sundered Eagle itself, not clear if it will return or not IIRC) but it has all the features of a domus magna, specially the largest collection of extremely cool magic items in the whole order of hermes. No political fopcus, but one of the most powerful covenants in the order.
Cheers
Xavi
The Rhine book is all about the Hermetic environment. You couldn't devote a book to mundane and hedge magician affairs and still have that level of Hermetic detail, both for reasons of space and for theme.
Ah how about Magic that destroys the Gift. 
:mrgreen: Great Idea where can I pre-order.
REALLY like this idea! That said, I'd like a little more story and a little less "scores at year 30 post gauntlet".
That is something I note with some interest...
I like it!
If its too narrow for a single book it would combine well with the earlier discussion of crime, criminals, and punishment.
I don't think it would be too narrow in scope. I'd like to see it organized like "Hermetic Projects." What is the crime? How does it run afoul of the code? What would the hermetic/mundane response be. Ideally, the book would provide SG's with details that could be used with magi who commit these crimes, are accused of them, or are on the quasitor side. This would satisfy my desire for more practical details about the code without nailing down too many specifics.
Then the technical term in infamous - or dead. Probably both.
That's what I think. If the one thing that makes you special is the Gift then the last thing you want is for every one to have it.
What happens when powerful nobles get the Gift? The holy roman emporer? These people have the resources to dominate the Order. What happens when the Pope and all his Cardinals get the Gift? Will the Order be forced to serve the Church?
What would the Order do if suddenly there were thousands of people were eligible to join. Are they all to be apprenticed? And by who? And what happens to those who do not get training?
All things considered, it might be best if you just Marched anyone who discovered how to give the Gift.
That's one way to look at it, but by no mean the only one.
What would happen if, suddenly, magi could decide whom they would take as an apprentice? Then having the Gift is no longer the key issue to determine who has power, knowledge and training are. And the Order of Hermes has a major advantage since they've had centuries to accumulate and record knowledge. Sure, now they would move to protect it (if they're smart). But it would lead to important leverage against those who crave that knowledge. The Gift doesn't give you that much power by itself.
On top of that, The Gift comes at a heavy price -- the social interaction penalty. Without Parma Magica (the great secret of the Order), all those new people with the Gift would have trouble maintaining their authority (if they have any to start with).
So it would certainly make for a different tone of Mythic Europe, but whether it would cause the Order to wax, wane or simply transform would depend on the saga.
While not explicitely stated, it's already there. There's also an official spell that kind of "damages" the gift - and it's considered "legal" as far as the code goes. It's the spell the Verditius use to make sure that magi who join their house after apprenticeship also have to rely on Casting Tools (Houses of Hermes: Mystery Cults).