I didn't have a problem with the cast of the recent films. I thought Groffud was especially good as Richards.
The only weak link I suppose would be Alba. She's nothing like the comic book character. But, I understand why.
The cast of the four heroes wasn't the problem. It was the overall corniness of the films. The only good action scene in them was the scenes with the Silver Surfer(who was FAR underpowered to begin with). Then, they ****ed up Galactus.
Now, you should recast Doom. Doom is in competition with Joker for the title of greatest villain in comic-dom and he was made to be more shitty in the films than General Grievous was portrayed in ROTS.
Make his costume an actual armor like in the comics. Give him his regular garments and don't try and make him look like an action figure. What's with the elctrical control. The man is a genius. Portray him as such.
Evans I thought, did a great job as the Torch, and Gruffud was pretty good as Reed.
Chiklis...not bad, got the demeanor right, but I always pictured the Thing as being somewhat bigger.
Alba? Nyet. Maybe Scarlet Johannsen, except she's a bit too young.
Doom...I did not like bobble-head McMahon, but at the moment I can't think of who'd be better.