But I would suggest, why not?
So bring in a Romulan villain in NEM.
General movie-going audience doesn't know the difference between new unknown Shinzon or backstoried Tomalak or anyone else for that matter. They'll only know from whatever is presented in the movie. So what difference will it make? It'll all be new to a general viewer.
Hardcore fans *will* know the difference between a previously unknown villain and a recurring character from the series.
So if the script calls for a Romulan villain, and if it's appropriate for the story, bring in an existing character.
The general audience won't know who Sela or Tomalak is? So what? NO ONE knew who Shinzon was, does that mean EVERYONE was alienated and confused? No, we all learn together during the movie. As with TWOK, good writing should be able to allay any backstory issues.