I've been getting so many good responses to this topic that I've decided to post my own idea:
MARVEL VS. CAPCOM 3 (in 2.5D)
CAPCOM VETERANS*:
Ryu (Street Fighter)
Captain Commando (Captain Commando)
Strider Hiryu (Strider)
SonSon (SonSon)
Guile (Street Fighter)
Hayato (Star Gladiator)
Rubyheart (Marvel vs. Capcom)
Amingo (Marvel vs. Capcom)
Batsu (Rival Schools)
Yun (Street Fighter)
Felicia (Darkstalkers)
Anakaris (Darkstalkers)
Dhalsim (Street Fighter)
Viewtiful Joe (Viewtiful Joe)
Ken (Street Fighter)
Saki (Quiz Nanairo Dreams)
Frank West (Dead Rising)
Eagle (Street Fighter)
CAPCOM NEWCOMERS**:
Dante (Devil May Cry)
Arthur (Ghouls & Ghosts)
X (Megaman X) to replace Megaman
Nathan Spencer (Bionic Commando)
Shoma (Rival Schools)
Jon Talbain (Darkstalkers)
Lou (3 Wonders)
Guy (Final Fight)
Haggar (Final Fight) to replace Zangief
Zack (Zack & Wiki)
Falcon (Power Stone)
Akuma (Street Fighter) with Cyber Akuma mode
Donovan (Darkstalkers)
Protoman (Megaman)
Kenji (Red Earth)
Chun Li (Street Fighter) with Shadow Lady mode
Okami (Okami)
Maximo (Maximo: Ghosts to Glory)
MARVEL VETERANS:
Spiderman (Spiderman)
Wolverine (X-men)
Cyclops (X-Men)
Dr. Doom (Fantastic 4)
Hulk (Incredible Hulk)
Venom (Spiderman)
Sentinel (X-Men)
Ironman (Ironman)
War Machine (Ironman)
Thanos (Ironman)
Rogue (X-Men)
Blackheart (Ghost Rider)
Silver Samurai (X-Men)
Saber Tooth (X-Men)
Iceman (X-Men)
Magneto (X-Men)
Gambit (X-Men)
MARVEL NEWCOMERS**:
DareDevil (DareDevil)
Captain America (Captain America) with U.S. Agent mode
Ghost Rider (Ghost Rider)
Mr. Fantastic (Fantastic 4)
Hate Monger (Fantastic 4)
Silver Surfer (Silver Surfer)
Moon Knight (Moon Knight)
Black Cat (Spiderman)
Punisher (The Punisher)
Thor (Journey Into Mystery)
Nick Fury (The Avengers)
Deadpool (X-Men)
Lizard (Spiderman)
Hepzibah (Starjammers)
Nightcrawler (X-Men)
Kraven the Hunter (Spiderman)
Green Goblin (Spiderman)
Howard (Howard The Duck)(
)
Hydra Soldier (Strange Tales)
*Meaning anyone who's been playable in a Vs. game before (including CvS and NxC)
**Includes veteran characters with added special modes