I had a wild idea......what about the big bad being.......LAL?
Yeah, THAT Lal - Data's first (only?) offspring attempt. If I recall from that episode, she positronic matrix began to break down (or somesuch) and Data said he reintegrated or transferred her into him so while she is no longer a separate entity, she will always be a part of him. While the nature of the transfer is.....poorly defined.....we could extrapolate it to be simple fact, impressions, perhaps even latent emotions - basically whatever a writer could want.
In Nemesis, Data transferred his mind/self/whatever into B4 - which never truly became a new version of him. But obviously enough survived that Data could appear again. Fast forward to Picard season 1. Data is somehow alive through a single atom of his positronic net......or something.....(I'll admit I only watched it twice and have forgotten the details since)......being......somewhere.
But what about the copy in B4's head? As far as I can recall, they didn't vaporize B4's head, it's just in a drawer at Starfleet in Okinawa. What if Lal is still in there? And what if she's......unhappy.....at being forgotten for three decades? I would be. And who's to blame for being forgotten?
Data, obviously - I mean when he got emotions, did he just stare at her picture and weep without giving a thought to restoring whatever is left of her?
Picard. Well, he "killed" Data and may have ended her chances of restoration. Heck, he even got an android body himself!
The rest of the crew. They all interacted with her and not a single person thought about her in 30 years.
Of course, for all of the above to be true and for her to truly feel betrayed, she would have to be fairly emotionally unstable, or have survived this long in a tangible form at all (even if that form is just data), but frankly, I think it tracks better than Tasha Yar surviving after all this time even after the destruction of Romulus. There's also a ton of wiggle room - add a little data corruption to whatever Data kept of Lal and you could easily have a hyper intelligent, super strong, psychotic, female android baddie.
I would even have her pulling the strings of a bunch of other baddies we've seen in Next Gen. The aliens that were abducting Enterprise crew members in their sleep, or the alien bug creatures from "Conspiracy", have her orchestrate a bunch of disconnected attacks before the crew learns who she is.
Of course my second preference would be Ro Laren. If she spent time in a Cardassian prison for her time in the Maquis, she may blame the Enterprise crew for never coming after her even after the Dominion War.
My third choice would be the female founder, but her going after the Next Gen crew at this point doesn't seem to fit. Have her be manipulated by Lal though......
ING KNOW "The Offspring" was made years before Isa Briones' birth) I admit I'd kind of like to hear a possible in-universe explanation as for why Lal didn't look like Soji/Dahj/Kore/etc.
