I always thought Scarlet Johanssen would've made a good Sue Richards. And while I wasn't keen on Chris Evans as the Torch at first (jeez, they couldn't even dye his hair blond, even after giving Alba eerie blue contact lenses?), I thought he virtually nailed Johnny's youthful cockiness. His Torch will be a tough act to follow, I think.
Chiklis wasn't bad as the Thing, except that I thought he was a little too small physically. Also, if they could make an in-movie action figure look like Kirby's Grimm, couldn't they have done same with Chiklis' makeup? Other than that, I thought Chiklis also did rather well in capturing the essence of Ben's personality.
Gruffud's Reed...? It wasn't bad, but...I dunno. I wouldn't mind seeing Reed recast. And as for Alba as Sue, or McMahon as Doom...definitley recast.
For that matter, I like Doug Jones, but the next Surfer should have a heftier musculature (eg Kirby's Surfer, FF#72).
Overall, I think the FF movies are very underrated, but a reboot certainly couldn't hurt.