I thought Shinzon was an interesting concept, and with more development and a better motivation he could have been a terrific villain. With all that said, I like your idea of putting the focus on the Romulans and eliminating the Remans from the movie entirely. The Romulans were my favorite villains in TNG, and I think a conflict between them and the Federation could have made for a far more interesting movie than what we actually got.
I would agree with WarsTrek1993 that Nemesis did have its moments, but IMO its many flaws overshadowed a lot of its good moments. I don't think Nemesis is nearly as bad as the critics and most Trek fans claim, but regardless it could have been much, much better.
Going back to how it could have been different, I like your idea of a military coup with an aggressive Romulan taking over Romulus and trying to wage war with the Federation. At the same time, I somewhat agree with T'Girl that the story would need a lot of fleshing out so that it didn't just turn into an action movie with the Federation vs. the Romulus. I'm sure that's what you intended, though, so cool idea.