So interesting!
I had heard the Maul/Grevious ideas before, but this actually isn't very bad. It adds depth, keeps a great character, and on paper, it's very cool, but it does raise some problems:
Why would Maul be a general if he was the bad-ass secret sith assassin from TPM? He was more of the quiet follower rather than a military commander.
Why would Palpatine spend so much time with Anakin? Why would he get Anakin to kill Dooku?
How do you make a reveal of Maul's resurrection? How do you hint to it throughout the film? How do you depict his second resurrection?
How do you convey that Vader and Grevious are one and the same when Grevious is spindly with no organs and much machinery, while Vader is clearly humanoid?
How do you include Anakin's "death" or fight with Obi-Wan without making it obvious that he's Vader?
Should you have Obi-Wan recognize Maul? If not, how to you interestingly convey who he is?
Why would Palpatine sink so much effort into keeping Maul if he has already replaced him with Dooku?
What do you do with Obi-Wan's fight with Anakin? What causes Padme's death? Does Anakin still cut the hands off of Windu? Does Windu confront Palpatine at all? If not, how does Palpatine become disfigured?
How much new footage will it take to add the Maul Resurrections story arc? how do you make it seem like "Maul"'s body was recovered from Utapau without actually showing it happen (as it actually didn't).
Now I know that sound like a barrage of negativity, but really I'm just wondering how on earth anyone could pull of such a massive change without anything short of inserting entire new sequences into the film (a feat that even funded professionals would have time with) and add in lines (making the whole attempt damn near impossible).
I say near impossible. If you made Maul speak a subtitled alien language in TPM and if you had Grevious speak the same language the same way and if you managed to film new sequences of Grevious's reveal in AoTC and his resurrection in TPM you could (with difficulty) make me believe that Maul = Grevious.
But Grevious = Vader?