Here is a list of all the characters you can unlock in the game. The entire bottom row of characters is dedicated for custom characters A-Z. There is a total of 132 characters in the game. (not including any future or current DLC.)
In order they appear on the list from left to right and their outfits in parenthesis.
Ironman (Heroic Age) (HeartBreaker) (Mark 1, 6, 7 ) (Mark 42) (Tony Stark) (Underwear) Hulk Spiderman (F.F.) Captain America (Classic) Mr. Fantastic (F.F.) Hawkeye Black Widow Wolverine (Cowl) Human Torch Thor (Classic) Cyclops (Astonishing) Jean Gray (Phoenix) Storm Beast Iceman Invisible Woman (F.F) Thing (F.F.) Nick Fury Maria Hill Agent Coulson Abomination Absorbing Man AIM Agent Aldrich Killian Ant-Man Archangel Arnim Zola ??? Beetle Black Bolt Black Cat Black Panther Blade Blob Bullseye Captain Britain Carnage Colossus Curt Connors Damage Control Daredevil Deadpool Destroyer Minifig Dr. Doom Doctor Octopus (Ultimate) Dr. Strange Doombot Doombot V Series Dormammu Drax Electro (Ultimate) Elektra Emma Frost Frost Giant Galactus Gambit Gamora Ghost Rider Green Goblin Green Goblin Ultimate (this is another character slot and not an outfit) Groot Gwen Stacy Havok Heimdall HERBIE Howard the Duck Hydra Agent Iron Fist Ironman Hulkbuster J. Jonah Jameson Juggernaut Kingpin Kingpin Henchman Kraven the Hunter Kurse Laufey Leader Liberty Loki Magneto Magneto Acolyte Malekith Mandarin (film) Mary Jane Watson Mastermind Mini Sentinel MODOK Moon Knight Ms. Marvel Mysterio Mystique Nightmare Nova Pepper Potts (Rescue) Polaris Powerman Professor X Psylocke Punisher Pyro Red Hulk Red Skull Rhino Rocket Raccoon Ronan the Accuser Roxxon Guard Sabretooth Sandman Sandman Goon She-Hulk SHIELD Agent Shocker Silver Samurai Silver Surfer Spider Woman Squirrel Girl Stan Lee Star Lord Superior Spiderman Super Skrull Symbiote Scientist Symbiote Scientist Hazmat Taskmaster Toad Union Jack Venom Viper Vulture Iron Patriot (War Machine) Wasp Whiplash Wizard
(Edit: I noticed while I was writing this that I am missing one guy hopefully somebody has an idea. Im pretty sure you unlock them once you get 250 gold bricks. I'll probably get there sometime tomorrow.)
It was Aunt May I was missing. You get her after you unlock Deadpool and do his last 3 missions (you also get Deadpool's bike, and Helicopter.) I did list Superior Spiderman. Parker, Banner (as well as Connors (Lizardman) and Gen. Ross (Red Hulk) do not get their own slots. Instead you can transform into them when you hold down Triangle.
Oh and I got 250 and 100% today, and there is no reward for it that I am aware of.