just so you know, here's the list of the confirmed starting characters, along with 2 dlc characters and a few rumord characters

on the Marvel side you've got:

CAPTAIN AMERICA
DEADPOOL
DOCTOR DOOM
DORMAMMU
HULK
IRON MAN
MAGNETO
M.O.D.O.K.
PHOENIX
SENTINEL
SHE-HULK
SHUMA-GORATH (DLC)
SPIDER-MAN
STORM
SUPER-SKRULL
THOR
WOLVERINE
X-23

and on the Capcom side you've got:

AKUMA
AMATERASU
ARTHUR
CHRIS REDFIELD
CHUN-LI
DANTE
FELICIA
HAGGAR
HSIEN-KO
JILL (DLC) resident evil 5's version
MORRIGAN
RYU
SPENCER
TRISH
TRON BONNE
VIEWTIFUL JOE
C. VIPER
WESKER
ZERO

Confirmed Boss Character:

GALACTUS

several Rumored characters include:

War Machine
Bison
Frank West
Chuck Green
Volnutt
Roll
Cyclops
Venom
Dr. Strange