The origin-montage opening worked very well in The Incredible Hulk. That could work. Except, of course, the FF aren't just superheroes; they're primarily explorers and adventurers, and Reed is constantly producing new inventions and discoveries with world-changing potential. That's something I'd like to see a movie get right -- a portrayal of the FF as a scientist/adventurer family that includes superheroics and world-saving as part of its routine, rather than just as superheroes. That's what makes them distinctive, and it would be worth seeing onscreen.
Good point. 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.
The origin can easily be told in a short flashback in a briefing room scenario or something.