Johnny Depp would have made a good 'senior elf' - Elrond perhaps? Added dimension to the character, certainly...
The ones I would really like to recast are Frodo and Merry.
Merry should be a posh, sensible and clean-cut actor. Nationality not important.
Frodo, a thirtysomething intellectual English actor.
If I have any more specific ideas I'll get back to you, Lommy....