Suggestions by @richman

Doomsday

as Marlon Brando in What if this actor played

Dormammu

as Alan Rickman in What if this actor played

Kang the Conqueror (Nathaniel Richards)

as Alan Rickman in What if this actor played

Kano (Mortal Kombat)

as Hugh Jackman in What if this actor played

Captain Boomerang

as Hugh Jackman in What if this actor played

Malekith the Accursed

as Vincent Price in What if this actor played

The ancient one

as Vincent Price in What if this actor played

Kraven the Hunter (Sergei Dimitri Symostivych Kravinoff)

as Peter Sarsgaard in What if this actor played

Venom

as Tony Todd in What if this actor played

Two-Face (Batman Forever)

as Val Kilmer in What if this actor played

Alfred Pennyworth

as John Lithgow in What if this actor played

Doctor Eggman

as Bob Hoskins in What if this actor played

Norman Osborn

as Orson Welles in What if this actor played

Kingpin (Wilson Fisk)

as Orson Welles in What if this actor played

Alfred Pennyworth

as Richard Harris in What if this actor played

The Mad Hatter (Jervis Tetch)

as Jesse Eisenberg in What if this actor played

The Joker (The Dark Knight)

as Robin Williams in What if this actor played

Ben Parker

as Robin Williams in What if this actor played

Freddy Fazbear

as Matthew Lillard in What if this actor played

Scooby-Doo (character)

as Matthew Lillard in What if this actor played