I think Shinzon should have been Picard's son.
Then this movie would have been a validation of what Picard did and said in GEN and a reason for Picard to try to save him. By making it his son its no less a reflection on Picard. But he has more motive to save a son than an evil clone.
I would have cut Shinzon's creepiness. I just don't think he needed to be totally evil. He could have just captured Picard to take non-death threatening blood transfusions from him. Maybe it would have been better for him to try to destroy Romulus and maybe Picard would seem nobler if he was trying to save the billions on a planet he doesn't really like. Maybe he would sacrifice his only son (and Data to do it).