Denise Crosby was in talks to appear in Star Trek: Legacy
Let me preface this post by saying we don't know anything about Star Trek: Legacy other than it has not been greenlit. We don't know if it is even still a possibilty. That didn't stop Denise Crosby from bringing the series up at a Tampa Bay Comic Convention [thepopverse.com], and it sounds like she was all for what Terry Matalas had in mind for her.
According to Crosby, she had at least one conversation with Matalas, who was the showrunner for Star Trek: Picard, and Matalas told her he'd "love to do a storyline with Sela." She went on to expound on Matalas' success with the franchise and wondered why he hadn't been given "the keys to the kingdom" since he so clearly knows how to run a Star Trek show.
There's no denying that Matalas made a smashing success out of season three of Star Trek: Picard, and he'd likely do the same for Star Trek: Legacy. That said, the Sela storyline was, for me, a bit over the top and was just a way to bring Crosby back into the franchise, especially since it was her idea. I may be on my own with this, but the entire plot was a bit far-fetched. On top of that, Crosby just didn't come across as a believable villian. Again, this is just my opinion, and I'm sure there are plenty of fans who would disagree with me.
Picard dealt with the Romulans in season one of the series, and Admiral Picard even had a relationship with a Romulan [Laris] so bringing a Romulan back from Star Trek: The Next Generation who was still an enemy would seem a bit like overkill, especially since there's no guarantee that Patrick Stewart would return as Admiral Picard.
If Matalas was going to bring Crosby back, he had a chance to do so in the final season of Picard which saw plenty of familiar faces return. But there wasn't a place for her in the storyline. And with everything that's been said about Legacy and the actors that want the series, it seems like bringing Sela back would simply be shoehorning Crosby into a story that has already been told.