Well frankly the idea that the FF were the "only" super heroes around or the X-Men as the only super powered beings around always struck me as somewhat odd in that they all came from the same "universe". I can understand the whole "well you can't mention Iron Man in a Fox movie" and I can take it to a point. Sure, they don't have the rights to the character, BUT that doesn't mean you can't name drop or casually mention that "Oh and Stark's been working on some exo-armor concepts and needs my input I'll be home before dinner Sue darling"
It's a name - big deal.
My dream FF movie would have it actually pick up with them an already established Super Hero team. Sort of like how Rise of the Silver Surfer started out - people knew them, they have their tower and basically we don't need to rehash the origin. Maybe tell it in cartoon/comic style in the beginning during the credits and leave it at that.
And I would actually not entirely dismiss the original two movies - hell what I'd find funny as anything would be Johnny and Ben watching Silver Surfer and going "Oh geez Ben could they make you look any stupider and c'mon that's NOT what Galactus looks like? What? You think people couldn't take a guy taller than Four Freedoms tower in purple and pink body armor?"
"Well at least they picked that hottie to play our Suzie Q."
"Dude, not cool. I can't even watch her anymore without seeing... shudder... my sister."
So that way you don't entirely have to find a new way to 'reboot' the story, just say that the original two were movies loosely based on the real thing. Which would totally be a very Fantastic Four thing to see.
Now as for casting, that's the tricky part - honestly I really did like Chicklis as The Thing and Chris Evans did a pretty decent Torch - buuut they need someone a bit more in the vain, funny and teen demo look for the nuTorch. Although I'd like to see someone who's maybe not well known but still not a total newcomer. So my pick for a new Johnny Storm would be Hunter Parrish from Weeds - he's blonde, can be serious but I think if he was given the direction to be more 'Johnny Storm' I think he could pull it off. He's worked around enough comedians and while he's never really done any stunt work - that's not the worst thing really.
Invisible Woman would be a bit tricky - a natural blonde would be preferable. Someone who's got a bit of an athletic build - which is probably why Alba was picked, she's got a decent body, and when she was on Dark Angel, she was used to stunt work and hey she's gotta look decent in a skin hugging bodysuit. But I think they need to be more selective in picking actors and actresses that actually LOOK like siblings. My dream Sue Storm would have been - if it was a few years back - Jeri Ryan. (Although I still think she'd make an awesome Ms. Marvel) but I think of the actresses out there today - Laura Vandervoot - could make an interesting late 20s Sue - she's like 28 so by the time filming wraps up she'd be close to 30 - which'd be decent enough. But at least she's got experience with super hero shows (Being Supergirl in Smallville) and she's got a decent enough build to wear an FF uniform
Now Reed would be a bit tough since you need someone who's older than Sue and Johnny, similar age to Ben - but also can pull off the genius role without looking like a total nerd. Honestly I'd love to see someone take what we had in the "old" Reed from the previous Fox movies - but with a stronger actor.