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).
Don't ask me :p If I ever get to doing my SW edits (probably after the Blu-ray sets are released, dunno... and when I will have LOTS of time), I, personally, am not going to hide the fact that Anakin's Darth Vader (I actually want to give the Saga a unity by making it look actually MORE like the 'Tragedy of Anakin Skywalker', instead of this tragedy being thought of in the last moment). But, anyway, regarding your questions.
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.
Because there were no Separatists that Palpy had to lead? Anyway, regarding Grievous character - we have the advantage of him having NO FREAKING mouth, meaning he can say WHATEVER we want... OR he can be silent most of the time, like Darth Maul, speaking only on business and when effecting certain orders.
Why would Palpatine spend so much time with Anakin? Why would he get Anakin to kill Dooku?
Because, regardless if we cut the 'Chosen One' crap or not (I'm not a PT-hater, but I don't like the Chosen One concept since it feels forced... mostly because it wasn't really thought of from the beginning, like 95% of Star Wars Saga :p Heck, even Vader wasn't Luke's father when the first SW was filmed... anyway, where was I... oh, yeah, regardless if he was the Chosen One or not, Anakin still is a Jedi of remarkable capabilities, more remarkable than in anyone else Sid could think of.
Now, come to think of it, I really don't have that much clear answers. Anyway, I think that not only it's impossible to make Grievous and Vader one character, but also implausible. However, we could ease the Maul = Grievous stuff by taking Sid's lines from RotS, pasting them into TPM and naming Darth Maul Darth Grievous (there's only one or two places in the whole movie where the name 'Maul' is uttered).
Regarding Anakin's Dark Side. If Anakin's a loose cannon, we could make it seem like Palps didn't want to make him an apprentice, but to go berserk on the Jedi, I don't know.
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EDIT:
What I'm more interested in, is it possible to make so that the whole Saga would NEVER show the same planet in two movies? (aside, MAY BE, from Tatooine).
See here:
EpI: Naboo, Tatooine, Coruscant
EpII: Coruscant, Tatooine, Kamino, Geonosis
EpIII: Coruscant, Utapau, Kashyyyk, Mustafar, Polis Massa (plus Alderaan, Tatooine, Cato Neimodia, Naboo, Felucia, Mygeeto, Saleucami make short appearances)
EpIV: Tatooine, Death Star (let's count it as a planet :p ), Yavin (Alderaan makes an appearance)
EpV: Hoth, Dagobah, Bespin
EpVI: Tatooine (AGAIN THE BLASTED! :) ), Endor, Death Star II (Coruscant, Tatooine, Bespin, Naboo in 2004 make short appearances, but I guess that's acceptable)
Do you think it's possible to make, let's say, Coruscant being only MENTIONED in EpI and putting the Council on Dantooine, for example? Or stuff like that? Make solo be held captive not on Tatooine, but on Nal Hutta (really, there's no reason why Jabba the Hutt HAS to be on Tatooine. It would make him a much more fearsome Crime Lord if everyone around the galaxy knew him :p ). You know, to make the universe seem VASTLY bigger.
But, most importantly... I really don't like the concept of the second Death Star (yeah, I know it's RotJ territory, but the PT has the biggest number of the same planets used in several films in a row, so I'll justask it here), I think one Death Star is enough. Do you think it's possible to change the DSII to a giant Imperial fleet? (or, I don't know... something).