I have given a lot of thought to this ever since I saw the Lord of the Rings Films, having recently read The Hobbit... My first thought was of the Dwarves. A few of them were amusing and a few of them were serous considerations of who could play the rolls...
Thorin:
Brian Blessed (Come on! You know he'd be perfect!)
Some other dwarves could be played by...
Dwalin:
Tony Robinson (Played like Baldric from Blackadder, obviously)
Balin:
Bill Baily (The musical scenes would be so much funnier)
Bombur:
Collin Baker (Because he's FAT)
Bifur:
Tom Baker (Because he is ACE)
Dori:
David Mitchell (Why not?)
Kili:
Julian Barrat (One part of the double act)
Fili:
Noel Fielding (the other part of the double act from The Mighty Boosh)
Dáin Ironfoot:
Billy Connolly (with a banjo)
Beorn:
Oliver Reed (after seeing him in Baron Munchausen, I thought he'd be good for the roll)
Bard the Bowman:
Christopher Eccleston (Because he SHOULD be northern)
Smaug:
Vic Reeves (Of course)
Thranduil:
Rowan Atkinson (No idea why, the concept just amuses me)
Drunken Elf:
Dylan Moran (Obvious, really)
Not entirely plausible. But it would be a darn fine film!