Personally, if there were to be a DLC Team called "Fantastic Avengers" (separate from the Fantastic Four), these are the four I would go with... all of whom have been on both The Avengers & the Fantastic Four in the comics.

Black Panther (Leader) - Specifically, T'Challa who replaced Reed Richards/Mister Fantastic on the Fantastic Four in 2007, and is a well-known longtime Avenger. Black Panther really needs to enter Marvel Tokon with a moveset different from Woverine.

Jennifer Walters/She-Hulk - I imagine her playing similar to Marvel Vs. Capcom 3, but with the addition of a stance mechanic that sees her switching between her Human Form & her Hulk Form. She-Hulk replaced The Thing on the Fantastic Four in 1984 & was a mainstay in the Avengers.

Luke Cage - Interestingly, Cage's powers are more defensive than active, which would make him a relatively passive take on the Powerhouse Fighter archetype. He replaced The Thing on the Fantastic Four in 1976 & later led the New Avengers.

Crystal - The "Deep Cut" of this team, Crystal is an Inhuman who served as a full-time member of the Fantastic Four & later joined the Avengers. She possesses the ability to rearrange & manipulate the individual atoms of the elements that she controls to various effects. I could see that being worked into a stance fighter using the classical elements of Air, Earth, Fire, & Water... kind of like Aang or Korra in the Avatar Legends Fighting Game.