Replacing Sidious with Grievous is an interesting idea. I had always liked the idea of trying to imply that Maul became Grievous (though admittedly that brings more problems too) so I’d not considered the idea of suggesting the two work together.
Well, Grievous IS the general of the droid armies and he is working in coalition Darth Sidious (the emperor). I don’t see any reason why he shouldn’t appear before EP3. Unlike OT, PT appears too disconnected with its own episodes anyway (a character appearing out of nowhere, like writers just thought him/her up and couldn’t wait to add him/her to the movie), this would help a bit with that.
Originally Grievous was chosen and trained by Count Dooku, under the orders of Darth Sidious, but since we take Count Dooku out with this plan, you would have to tie Grievous to Sidious directly. (and any other sith apprentice Dooku has goes to Sidious as well, screw the stupid “rule of two” as well, if it wasn’t on the OT, I don’t consider it canon.) By the way, I am really for adding some stuff from animated series (like sith warriors and stuff) and from KOTOR games (republic related stuff) but that would require a lot of CGI work. (these movies already has many terrible CGI scenes)
Also one more idea, in EP3, how about Anakin doesn’t slaughter bunch of little kids but some other group of people Palpaline doesn’t like? (Like two or more Jedi consul members or Republic soldier or something) There should be something to show he is turning evil, but directly slaughtering children seems too far a jump. (I mean, he might have done that as Darth Vader, but he was still Anakin when he slaughtered those children. It just doesn’t make any sense whatsoever, how did Sidious convinced him to do it? Anakin wasn’t completely evil yet. At first, he did what he did thinking it was justified after all. How did he justify murdering children all of the sudden?)
Lastly, this is what I meant when I talked about Grievous sounded like a cartoon villain in EP3:
https://www.youtube.com/watch?v=HeNhyWhZ_oc
His voice actor in Clone Wars (2003) cartoon did an awesome job, he actually sounds intimidating. But in Episode 3, his voice actor did a terrible job and sounds like a cartoon villain. (with a weird accent and a voice very difficult to understand without subtitles)
Lastly, I think Grievous should have red lightsabers… Who is with me?