I don't see what changing Grievous to Maul would've done... I mean, if Maul hadn't been in TPM, then it would've been cool to see him as a new character in RotS... but it would've been dumb to bring him back to life or something after TPM. It would've been even cornier than Grievous, and fans would just complain about Lucas putting Maul in because he's so popular, like what he did with Boba Fett in AotC.