I like Tom Hardy, but I'd have preferred to see James Marsters in the role--but even better would have been Patrick Stewart with some X-Men 3 CGI to make him look younger
A scarred version of Patrick Stewart would have been it.
Shinzon was supposed to replace him. So let him be the same age. But since things went wrong and he fought in the Dominion war, let him have one half of his face burned, one arm crippled or something like that.
Shinzon should have the genetically programmed drive to overthrow the Federation and to follow the orders of his programmers, no matter how irrational it might seem at the time. And throughout the film we see him fight against himself because of that. Picard recognizes this and tries to appeal to his human side. But when his good side wins the battle, it's already too late for him and he gets killed.
Ditch the B4 story, because two clone stories is too much.
The film should have delved into the clone infiltrator story instead. Let there be an entire group of Starfleet officials who actually got replaced and start to attack the Enterprise. Let that be the moment where Shinzon overcomes his programming and does the RIGHT thing by trying to save Picard, getting killed in the process.
It would basically be a combination of the Picard/Shinzon and Data/B4 plots. Shinzon's internal battle, B4's infiltration, and Data's scrifice.