I think Shinzon made a lousy substitute for Khan, but the character in concept could have made for a good Darth Vader, and that story would have been a good way to return to TNG's roots. In the pilot, Q urges Picard to use the ships weapons against the mysterious threat over Farpoint, but Picard opts for nonviolent solution, setting the stage for the series. It could have been reflected in the film with Picard, like Luke Skywalker, finding a way to reach and redeem the enemy that everyone is telling him to kill. Preferably in a less cheesy way than Return of the Jedi
One of NEM’s myriad flaws is that it seems to forget Picard’s sobbing in GEN that “There will be no more Picards.” Shouldn’t there have been signs of hope, love, relief, something positive on his discovery that there actually is another Picard? It would have been a satisfying way to wrap up the TNG saga if a redeemed enemy became the vehicle by which the Picard family got new life.