I'll throw my hat into the ring as a potential GM. Before I talk about what I would like to run, a few caveats:
1) I've never used Mumble before, but it can't be that difficult, can it?
2) Never roleplayed over voicechat before, but I have close to 35 years experience runing pen & paper games, so I guess I should be able to handle a voicechat game.
3) (This may be the biggest problem) As I am at GMT+1, best time for the game sessions would be weekends, starting at 10am or 11am EST, given a session length of 5 to 6 hours.
4) I prefer smaller groups. 3 players, maybe 4, but I would also be happy to run something with just 1 or 2 players. Smaller numbers are fine with me, larger groups not so much.
That said, what I could offer as the actual game would be (with a strong preference for option A):
A) "Supers! Revised Edition" Supers is a superhero game that I would like to give a try. The rules are fairly easy to learn (in my opinion), and character creation can be relatively easy, once you have some character concept, some "archtype" to base your character on. I walked one of my freinds through character creation and it took about an hour to come up with a finished character. No guarantee it will be that swift for every character concept, but compared to some other systems I consider that still pretty fast.
The setting would be a fictional city in the USA, but not part of the Marvel or DC universes, i.e. an original setting with original characters (as far as anything based on several decades of comics can be original
B) Feng Shui. Currently not my preference, but I might run it, if there is enough interest and nobody likes superheroes all that much. Feng Shui is a game inspired by Hong Kong action flicks and wuxia movies that mixes elements from fantasy, modern, and sci-fi genres. Again, the rules are fairly simple, but the setting background is perhaps a bit more complicated. On the other hand, player characters don't have to know all that much about the setting in the begining.