Now don't get me wrong I don't think a remake is a good idea nor do I want to see it happen-this is just for fun!
Anyway here's my picks:
Adult Simba-Joseph Gordon-Levitt
Adult Nala-Mae Whitman
Scar-Jason Isaacs or Ralph Fiennes
Mufasa-Michael Dorn, Keith David, or Dennis Haysbert (for some reason I thought he was Forrest Whitaker which is why I put him there)
Timon-Adam Sandler (I know this sounds like a weird choice at first but if he uses his normal voice and doesn't go into an annoying voice (which is what he usually does for voice acting) it might work because I can see his normal voice fitting a little egotistical fast talking animal but only if he talks a little faster)
Zazu-Simon Pegg or Ricky Gervais
Sarabi-Moira Kelly (Why not have one of the original cast members play a new character?)