The really sad part of the Marvel Universe movies is that because of corporate crap, Marvel DOES NOT own the rights to the Fantastic Four or Doctor Doom. 20th Century Fox has the rights to these characters and the X-Men too I believe. I would be so great if Doom could show up at the end of the Infinity War to fight Thanos. He's certainly on his power level and Doom has fought for the Infinity Gauntlet before and is totally fine with slapping down any cosmic entity he ever faced.
Plus how cool would it be to see Wolverine join the Avengers like he did in the comics. Those issues are some of the greatest superhero stories ever. And now Spider-man will be leading the Avengers this fall in the comic books. I like Spidey as part of a team like when he joined the Fantastic Four. He's unofficially always been the FIFTH member of the team.
Marvel should just pony up some of the money they make and just buy back the rights to the Fantastic Four so they can finally make a good story with them There are so many great Fantastic Four adventure.