IMO, Shinzon should have just been a mysterious new Praetor. Not Picard's clone. There's a bit earlier in the movie where the Enterprise crew get this mission from Janeway about the upheaval in the Romulan government, and they're having this meeting and going "Hey, we don't know jack shit about this new guy". And there's a real sense of mystery there. Who is Shinzon? What are his intentions? What could it mean for the future of the Alpha Quadrant? You know, follow that up and there would be a great political thriller somewhere in there.
But instead we got Dr. Evil.