I like it being Fantastic Four. Keep things the way it is. Sue & Ben get married. Sue Grimm is the leader as Invisible Woman. Ben Grimm takes the name Wonder Man. Johnny Storm is The Human Torch. And my pick for the new fourth member would be a new Ultimate character. Somebody who keeps the others the way they are today, and doesn't change there stories. But this character gives what I think is essential for a Fantastic Four book, the THING character. It isn't Fantastic Four without The Thing. So I say introduce She-Thing to the team. Although I say take a page out of the current 616 FF comic, and rename her Miss Thing. Miss Thing sounds better than She-Thing. This new Ultimate character can be anyone from the UU, a Ultimate version of a 616 character, or a new character for her human form. And for a new Ultimate twist, she could morph from human to Miss Thing at any time. Then this way fans don't get the same "Thing" story of Ben Grimm's feeling sorry for himself story, the current Ultimate Fantastic Four stories are still moving forward, and fans still get a THING for their Ultimate Fantastic Four.
I vote in the way of tradition and nostalgia, with some new twists for a new Ultimate Fantastic Four.