Originally, the concept was that they were not even going to have costumes. The costumes were only added after the poor sales of the first couple of issues.
Well, at the time FF came along, Marvel didn't really do superhero comics, but focused more on horror/SF, romance, Westerns, and the like. So the first couple of issues of FF were written in more of a monster-story vein. The male characters' powers are all a little freakish, Ben is explicitly turned into a monster, and the first issue is largely about battling the Mole Man's monsters, with the second being a battle with the monstrous-looking "Skrulls from Outer Space!" So the original premise was very much in the vein of sci-fi B-movies of the '50s and '60s -- radiation transforming astronauts into bizarre entities, giant monsters coming from beneath the Earth or living on Monster Island, Red-Scare allegories of disguised alien invaders in our midst. So reorienting it toward costumed heroes battling supervillains was more than just a change of clothes, but a shift of genre as well.