There's a lot of characters in the previous games. Not everyone's going to make the cut, and that's before new ones are added, such as some of the newer Champions and Agents of Atlas characters.
A-Bomb
Abomination
Absorbing Man
Adam Warlock
Agent Carter
Agent Coulson
AIM Agent
Aldrich Killian
Alex Wilder
Amadeus Cho
America Chavez
Ancient One
Ant-Man (Hank)
Ant-Man (Scott)
Ares
Arkon
Arnim Zola
Archangel
Atlas
Attuma
Aunt May
Baron Mordo
Baron von Strucker
Baron Zemo
Batroc
Beast
Beetle
Bengal
Beta Ray Bill
Black Bolt
Black Cat
Black Goliath
Black Knight
Black Panther
Black Widow
Blackout
Blade
Blazing Skull
Blob
Blue Marvel
Bowman
Bruce Banner
Bucky
Bullseye
Butterball
Captain America (Steve)
Captain America (Bucky)
Captain America (Sam)
Captain America (Peggy)
Captain America (2099)
Captain Britain
Captain Marvel
Captain Universe
Carnage
Cassie Lang
Charlie-27
Chase Stein
Chipmunk Hunk
Citizen V
Clea
Cloak
Cloud 9
Collector
Colleen Wing
Colossus
Corvus Glaive
Cosmo
Cottonmouth
Count Nafaria
Crimson Dynamo
Crossbones
Crystal
Cull Obsidian
Cyclops
Dagger
Damage Control
Daredevil
Dark Elf
Darkhawk
Darkstar
Deadpool
Death Locket
Deathlok
Demolition Man
Destroyer
Detroit Steel
Devil Dinosaur
Diamonback
Doombot
Dormammu
Doctor Doom
Doctor Octopus
Doctor Strange
Doctor Voodoo
Dora Milaje
Drax
Dum Dum Dugan
Ebony Maw
Echo
Egghead
Ego
Electro
Elektra
Emma Frost
Enchantress
Executioner
Falcon
Fandral
Finesse
Fin Fang Foom
Firebird
Forbush Man
Frost Giant
Galactus
Gambit
Gamora
Gargoyle
Gertrude Yorkes
Ghost Rider
Ghost Spider (was in Lego MSH2 as Spider-Gwen)
Gorgon
Gorilla Girl
Gorilla-Man
Grandmaster
Green Goblin
Green Goblin Ultimate Universe
Green Goblin 2099
Grim Reaper
Groot
Gwenpool
Gwen Stacy (616 was in MSH1, 65 in MSH2)
Hammer
Hammerhead
Happy Hogan
Havok
Hawkeye (Clint)
Hawkeye (Kate)
Hazmat
Heimdall
Hela
Hellcat
Hellcow
HERBIE
Hit-Monkey
Hobgoblin
Hogun
Howard the Duck
Hulk
Hulkling
Human Torch (Johnny)
Human Torch (Jim)
HYDRA Agent
Hyperion
Invisible Woman
Iceman
Inferno
Iron Fist
Ironheart
Iron Man
Iron Man 2020
Iron Monger
Iron Patriot
J Jonah Jameson
Jack of Hearts
Jane Foster
Jean Grey
Jessica Jones
Jimmy Woo
Jolt
Juggernaut
Justin Hammer
Kang the Conqueror
Karolina Dean
Kid Colt
Kingpin
Kingpin Henchman
Killmonger
Koi Boi
Komodo
Korvac
Kraven the Hunter
Kurse
Lady Spider
Laufey
Leader
Lightspeed
Lizard
Lockjaw
Loki
Lorelei
Luke Cage
M-11
Mach X (was Mach 5 in Lego Avengers)
Magneto
Magnitron
Malekith
Man-Ape
Mandarin
Mantis
Maria Hill
Martinex
Marvel Boy (was Protector in Lego Avengers)
Mary Jane Watson
Mastermind
Maximus
Medusa
Melter
Meteorite/Moonstone
Mighty Destroyer (golden age)
Militant
Mindless One
Mister Fantastic
Mister Hyde
Mister Negative
Misty Knight
Mockingbird
MODOK
Molly Hayes
Moon Boy
Moondragon
Moon Girl
Moon Knight
Morbius
Morgan le Fay
Ms. Marvel
Mysterio
Mystique
Nebula
Nick Fury Jr
Nick Fury Sr
Nico Minoru
Nightmare
Night Nurse
Nosferata
Norman Osborn
Nova
Odin
Okoye
Old Lace
Pepper Potts
Peter Parker
Phantom Rider
Phoenix
Polaris
Poundcakes
Power Man
Professor X
Proxima Midnight
Psylocke
Punisher
Pyro
Quake
Quasar
Quicksilver
Radioactive Man
Ragnarok
Ravonna
Rawhide Kid
Red Guardian
Red Hulk
Red She-Hulk
Red Skull
Red Wolf
Reptil
Rescue
Rhino
Rick Jones
Rocket Raccoon
Ronan the Accuser
Ronin
Roxxon Guard
Sabertooth
Sandman
Scarlet Spider
Scarlet Witch
Scorpion
Sentinel
Sentry
She-Hulk
SHIELD Agent
Shocker
Shroud
Shuri
Sif
Silk
Silvermane
Silver Samurai
Silver Surfer
Skaar
Songbird
Sparky
Speed
Spider-Girl (in Lego Avengers, this was the Ultimate version - I'd substitute Anya instead, her powers are the same).
Spider-Ham
Spider-Man (Peter)
Spider-Man (Miles)
Spider-Man Noir
Spider-Man 2099
Spider-UK
Spider-Woman
Spitfire
Squirrel Girl
Stan Lee (use archive sound clips from the previous games)
Star-Lord
Stingray
Storm
Striker
Super Adaptoid
Supergiant
Super Skrull
Superior Spider-Man
Surtur
Swordsman
Tactical Force
Taserface
Taskmaster
Techno
Terminatrix
Thanos
Thing
Thor Odinson
Thor Girl
Thor (Jane)
Throg
Thunderstrike
Tic
Tigra
Tinkerer
Toad
Tony Stark
Totally Awesome Hulk
Trauma
Triton
Two-Gun Kid
Ultron
Ulysses Klaw
Union Jack
Uranian
Ursa Major
Valkyrie (Brunnhilde)
Valkyrie (MCU)
Veil
Venom
Venus
Viper
Vision
Viv Vision
Volstagg
Vulture
War Machine
Wasp (Janet)
Wasp (Hope)
Wasp (Nadia)
Wendigo
Whiplash
Whirlwind
White Tiger (Ava)
White Wolf
Wiccan
Winter Soldier
Wizard
Wolverine
Wonder Man
Wong
X-Ray
Yellowjacket
Yondu