Evil twins and more evil twins...TNG already had an evil twin Logan/Berman/Spiner.
Combine Shinzon and B4 and you get...Lore. Couldn't have been worse. He frees Sela from the Reman mines, tempts the Romulans with Borg technology to get access to the Scimitar, replaces their crew with Remans and there you have it. Sounds dumb? Still better than Nemesis.