I agree with maddog00 on seeing Invisible Woman as more of a motherly type figure. I could definitaly see Charlize Theron in the role.
correct me if im wrong, my knowledge of the fantastic four is based on the cartoon (that chaltab mentioned and some people never heard of) and the original movie, but Mr. Fantastic and Invisible Woman are in that 30's age range (and married) The Thing is the same age as he's like Mr. Fantastics best friend. And (from the movie) Human Torch is Invisible Woman's younger brother (so he's about 20's and has a more reckless attitude).
I think the problem with The Thing is that he's hard to do as a live action character and get the look right with still having the moveability in the costume required. I mean, they could have gone CG but wouldnt you prefer a live actor with makeup than a CGI inserted Thing?
-Darth Simon