it could be done. there's room for another meant-to-be-killed-by-obiwan 'villian' in ROTS. grievous.
what the heck is he anyways? half droid, half .. maul?
and why is he so "grievous"? perhaps he was (nearly) killed by obiwan, and then, came back for more?
THAT would be a great story.
--> THATs how you bring back Darth Maul : tell the story of how he was turned into Grievous. include saga-relevant plot points, like the one where cyborgs are less force-sensitive than they were before they 'died' -- this would explain why vader had less control of the force, once his body became half-machine (which was touched on somewhere in the EU, although I must admit, I'm not sure where). that would be an awesome backstory. it would make BOTH grievous AND Maul more 'relevant' as characters; together, as one character, they would be Obi-wan's personal nemesis. the tragedy of the character would be, the Cost of Revenge. also, if Maul had been somehow 'salvaged' and turned into the grievous cyborg, then this would foreshadow anakin's fate, when he gets turned into a cyborg himself at the end of ROTS -- it would show how palpatine doesn't let his apprentice die, he'll turn you into a cyborg first, and you will serve him even after 'death'.. (which drives home the idea of how anakin "died" and was replaced by vader, so old Ben WAS right, from a certain point of view. this could all be reinforced, if Maul was really the Grievous cyborg) ..and Maul would have a purpose now: to foreshadow vader's fate.
it could be done ;)
it wouldn't even change the movies in any way. Maul is still Maul; he still gets cut in half. Grievous is still grievous, he still coughs and gets shot in the chest.
--> just insert a flashback episode between ep1 and ep2, where Maul gets rescued/resurrected and stuck into a hideous android body, because palps won't give up his pet project. he'd be like robocop... but the SIth Version of robocop. ;)
every character needs a backstory, afterall : this is how I've always imagined Grievous anyways -- he's the resurrected corpse of DArth MAul.
they both had the same "Purpose" : they were cannon fodder for obiwan's lightsaber :-\ (they might as well be the same character).