I agree it would have been cool if Tomalak could have been in Nemesis. Would have added to that connection to the series. It would have even been a more meaningful gesture on the part of the Romulans, if Tomalak had joined forces with Picard to stand against Shinzon.
I also think it could have been cool to have Sela involved somehow in the creation of Shinzon, but the problem is you have Sela, a doppelganger of Tasha Yar, in a movie that already has Picard's and Data's doppelgangers. So that was never going to happen.
Sadly, as it was, Nemesis dropped the ball on making connections to the series as it was. They completely ignored the fact Data had another brother, Lore, and he wasn't even mentioned. They acted as if B4 was the first double of Data ever seen before.