Turin - Tom Baker (He's far too jolly for such a role) Although I would like to see him proposing to Niënor by saying, "Would you like a Jelly Baby?"
Mim - Christopher Eccleston (a Northern Dwarf would be funny, though)
Morgoth - Sylvester McCoy (I just can't imagine Morgoth with a Scottish accent. Again, he's far too jolly for the role)
Feanor - Patric Troughton (Erm... Let's pretend he's not dead. He was always hyper active, happy and dressed like a tramp. However, he could play an angry character and had spells of Feanor-type arrogance "You'll regret it if you kill me! I'm a genius!")
Glaurung (voice) - John Pertwee (As much as I would LOVE to head a Dragon speak so politely and like an English Gentleman, I suspect it would distract from the character's desired effect)
Beren - Anthony Ainley (Although, at his peak, he was a talented and diverse actor (the Master's many disguises being a good example), this is one role I don't think he'd suit. Although, the scene with Carcharoth may be amusing, "I have a Silmarill! I am Beren and YOU WILL OBEY ME!")
Elwing - Elisabeth Sladen (I some how can't see her turning into a bird so readily
)
That's all I can think of right now...