I'm probably going to be shot for suggesting this, but here goes.
Vin Diesel as Magus.
'Aims''Rethinks' Maaaaaybe...No.
Keanu Reeves as Glenn
Crono... I don't know. PEE WEE HERMAN???????????????????
Peewee Herman as Anyone
No.
Marle- Dakota Fanning
Lucca- Emma Watson
Yes.
What about Ozzie, Flea, and Slash? Ozzie Osbourne should be a character.
Heh, I would personally love it if the real Ozzie Osbourne, Slash, and Flea had cameos in Magus' Keep. Maybe as lackeys of their ingame counterparts, heh.
Chrono- Shea Lebuff
Marle- Paula Abdul (cause they're both bimbos)
Lucca- Jennifer Love Hewitt
Frog- Cary Elways
Ayla- Keira Knightley
Robo- a computer
Magus- Laurence Fishburn?(if we're doing voice actors.)or Tom Cruise
Gaspar- Ian McKellen
Shia, Paula, Cary, Lawrence, and Tom? No. Jennifer and Keira? Maybe, but I don't how well Ms. Knightly could play Ayla. Ian McKellen? I'm going to assume this is the guy who played Magneto in the two Xmen movies (The third does not exist), so maybe.
I fully support Depp as Flea and Adrian Paul as Magus
If we're talking voice actors however...
Bill Murray as Toma (Dunno, just fits my idea of Toma's voice)
Christopher Lloyd or Alan Rickmon as Belthasar
Sean Connery as Melchior
Microsoft Sam as Robo's default voice. Or Stephen Hawking because he needs to be in a movie.
Mark Hamill as Bekkler (Imagine halfway between TAS Joker and The Gargoyle from The Incredible Hulk)
Will Wheaton as Fritz. (Yet another case of playing Chrono Trigger while watching The Big Bang Theory affecting my judgement)
Tim Allen as Taban for giggles (No, not really, although the idea of Taban saying "More Power!" while inexplicably hammering Lucca's machine brings a grin to my face.)
All I've got for now.