Jeffrey Dean Morgan
Morgan proved in Watchmen that he can power even the most dislikeable character with charisma. So filling Mr Fantastic’s rubbery goody two-shoes shouldn’t be much of a stre… No, not falling for that pun.
Why not him:
Morgan’s still not much of a household name (Watchmen is sold more on the characters than the stars), so the studio may not select him. Still, aside from the Dalmatians movies and (whisper it) King Arthur, who knew who Ioan Gruffudd was Stateside before FF?