Characters in italics are available. Others, as stated, are taken. Take your pick :)

Taken Characters:

Tony Stark - Iron Man. (Taken)
Steve Rogers - Captain America.  (Taken)
Thor Odinson.   (Taken)
Natasha Romanoff - Black Widow. (Taken)
Frances Barton - Female Hawkeye. (Taken)
Loki (taken) 
Victor Von Doom. (Taken)
Sif.    (Taken)
Sigyn. (Taken)
Tarene.   (Taken) 
Jane Foster. (Taken)
Mystique.   (Taken)
Charles Xavier.  (Taken) 
Amora the Enchantress.
Clint Barton - Hawkeye. (Taken)
Erik Lensherr. (Taken)
Johnny Storm - Human Torch.  (Taken)
Sue Storm - Invisible Woman. (Taken)
Phil Coulson.  (Taken)
Domino.  (Taken)
Peggy Carter.  (Taken)
Darcy Lewis. (Taken)
Fandral.  (Taken)
Storm.  (Taken)
Logan - Wolverine.  (Taken)
Remy LeBeau - Gambit.  (Reserved)

Avengers:

Bruce Banner - The Hulk.
Peter Parker - Spider-Man.
Carol Danvers - Ms Marvel.
Logan - Wolverine. 
Janet Van Dyne - The Wasp.
Jennifer Walters - She-Hulk.
Greer Nelson - Tigra.
Jessica Drew - Spider-Woman.
Bobbi Morse - Mockingbird.
Pietro Maximoff - Quicksilver.

SHIELD Agents:

Nick Fury.
Maria Hill.

Villains:

Justin Hammer.
Johann Schmidt - Red Skull.
Carnage.
Venom.
Jaime Slade - Phantom Rider.
Galactus. 

Asgardians:

Volstagg.
Odin.

Fantastic Four:

Reed Richards - Mr Fantastic.
Ben Grimm - The Thing.

Civilians:

Virginia Potts -  Pepper.
James Rhodes - Rhodey.
Gwen Stacy.
Betty Ross.

X-Men:

Any are welcome.

Other Characters.

Felicia Hardy - Black Cat.
Wade Wilson - Deadpool.
Frank Castle - The Punisher.