The three most likely choices for Avengers are probably Fraction, Loeb and Hickman, in no particular order.
In terms of preference:
Hickman > Fraction > Loeb
I can't see Waid, Brubaker, Parker, Gillen, Gage or Slott taking over the main Avengers title, as much as I would love it to happen. New Avengers or Assemble, perhaps, but for one reason or another I don't think any of them are likely choices.
I can see Fraction leaving one of the solos he writes in favor of becoming the writer of the company's top book. He has written Iron Man for a LONG time, maybe he already intended to leave anyway.
I REALLY REALLY REALLY hope that doesn't happen. Just as much as I hope Bendis doesn't move to the X-men franchise.
Top 3: Batman, Wonder Woman, X-force
I would welcome a soft reboot, something along the lines of Heroes Return, which relaunched a number of titles to #1 but didn't erase any continuity. The Avengers line could use a tidy-up.
I would hate a hard Crisis-style reboot that altered everybody's backstory, powers, personality and/or costume.
It'll probably be Fraction taking over. Though with multiple Avengers books it may be entirely possible that we get multiple Avengers writers.
Mr. Brubaker will probably stick with Cap and Winter Soldier...he's flat-out admitted that team books are not his strong point.
Hickman would probably be my preferred choice of those available, but I don't think his sales are on the same level as Loeb or Fraction.
Of those not mentioned yet: I'd like to see Christos Gage get a shot. I've liked pretty much everything I've read by the guy so far.