it's somebody that has to be dramatically updated. It can't be campy. The FF can be the more "serious" of the "cosmic" franchises-- the other being the more comedic Guardians of the Galaxy.
Also, problems need to be solved beyond an explosion at the end.

If it's Mole Man, then the writers have to explain what subterranea is and why it hasn't shown up at all until 2021.
If Mole Man is used, hopefully he won't be a dwarf. I also don't support Danny DeVito in the role.