I like the idea of Cathrine Zeta Jones to play Melian...but I think that Monica Bellucci would be a good choice to. The only one I think would be perfect for the part as Luthien is of course Liv Tyler...but she is already Arwen so I don't know if that is going to work.
I have also read the tale of Beren and Luthien several times, it's very dear to me so I would be very sad if it wasn't done properly.
Aftenstjerne