NOTE: I know some of these have been used in fancasts before, but they are suited for the role.
Mr. Fantastic/Reed Richards - Jon Cryer (Two and a Half Men)
Human Torch/Johnny Storm - Aaron Paul (Breaking Bad)
Invisible Woman/Sue Storm - Leslie Bibb (Iron Man)
Note: I know she is actually already in the MCU as the reporter in Iron Man 1 & 2. However, it is some what a "throw away" character and if other people (Matt Gerald & Vin Desiel) can play more than one character...why not?
Thing/Ben Grimm - Mark Valley (Boston Legal)
Note: This one I owe complete credit to a fancast by @KingULIK . I couldn't think of ANYONE and saw Valley on his and thought it was a great idea.
Dr. Victor Von Doom - Mads Mikkelsen (Hannibal)
Mole Man - Danny Devito (It's Always Sunny in Philadelphia)
Annihilus - Willem Dafoe (Spider-Man)
Norrin Radd/Silver Surfer - Zachary Quinto (Star Trek)
She-Hulk/Jennifer Walters - Gina Carano (Deadpool)
Namor - Jonathan Rhys Meyers (The Tudors)