Also, Pixar's movies aren't exactly kiddies stuff.
SPOILERS!
Nemo's mother is killed in the beginning of the movie. The father wont let his child out of his site because he saw his wife and children murdered (off camera).
The Incredibles deals with fidelity, mid life crisis, and other adult things.
Toy Story 3 was so sad at the end, the toys almost burn to death and Andy has to give up his childhood.
And UP!
Up was about growing old, death, loss of loved ones, the inability to have children.
Pixar has never shied away from adult themes that are a little mature for younger viewers.
So they can handle themes like a young child who has lost his parents and must live with his aunt and uncle, until his uncle is killed when he is a teenager because the teen didn't stop it when he had the chance. That Spider-Man concept is as mature as Up and the Incredibles, and Finding Nemo.
If Pixar did a FF movie I would want it live action, and that plus the already different back stories (the FF getting their powers and how they are all related to each other, while the Incredibles just have powers and are more of a traditional family), the movies would be different enough in my opinion. Enough to let the public not think they are just ripping off the Incredibles or whatever.
The main reason I would want Pixar involved in ANY Marvel property is their ability to focus on stories. They make such great stories.
I would rather then animate something like the Inhumans though, because they could make such a great movie with Kirby influence all through it, brought to life with in Pixar quality. It would be awesome.