I sort of agree about Shinzon. I'd remove the clone stuff, but keep the Reman thing. There's a scene early on in Nemesis where we're given an intriguing picture of what Starfleet knows about the new Praetor: absolutely nothing.It would make for a more intriguing villain to make him analogous to Kim Jong-un; somebody who comes into power as an unknown quantity, and therefore the story becomes about how this could shift the balances of power in galactic politics. Who is this Reman guy? What's his story? What are his intentions towards the other galactic powers? How does his coming into power unexpectedly change the Romulan Empire? Suddenly, he's a much more interesting villain than the one we got in the actual movie.
What scuppers him in the actual movie is that once we get into all that I-Was-A-Clone-Of-You-Who-Got-Abandoned stuff, suddenly the character loses that ambiguity, that mystery, that enigmacy that the early parts of the movie kind of brought up. Instead of being about Shinzon and how his elevation to power affects the rest of the galaxy, the story instead becomes simply about Shinzon and Picard. Strangely enough, he stops being an interesting villain precisely because it becomes a personal story (for Picard), instead of being an impersonal one (about Shinzon).
Unfortunately you can't do this though. The small change to the character would completely alter the entire point of the movie. Regardless of how you feel about the film, this is too much.
Also, this might work for people who care more about galactic politics more than the characters, but I'm not among them. And I don't think most common moviegoers would have been interested either. Keeping the story a Picard story makes it a better movie. If it's just about some random bad guy that now controls the Romulan empire, who gives a crap about that?
Respectfully, this kind of change to the character would only make a week movie worse.