We haven’t seen Thomas Riker, William T. Riker’s clone on Star Trek: The Next Generation, since Thomas turned himself into Cardassian authorities.
Appearing on an episode of Star Trek: Deep Space Nine “Defiant”, Thomas attacked a Cardassian outpost because he wanted to expose their secret activities in the Orias system. That didn’t work out so well, and the last we knew of him, he was headed to a Cardassian prison.
Star Trek: Lower Decks may have released him from that prison, though, as in “The Inner Fight,” Starfleet is hunting for ex-officers that may be recruited by this season’s villian, Nick Locarno (voiced by Robert Duncan NcNeill). Captain Freeman has a list of “at risk” civilians and ex-Starfleet officers who might join up with Locarno. And Thomas has already proven that he can be ethically swayed.
Thomas Riker was given life in prison, but Major Kira promised to return for him.
We don’t know much about Cardassian prisons, but they can’t be easy to break out of, which means, somehow, over the last thirty years, Thomas has managed to be set free. Did Kira keep her promise? That’s not known at this time, but if Jonathan Frakes makes a return to Lower Decks, we could get more answers about the elusive former lieutenant who once wooed Deanna Troi.
It’s would be fitting for Lower Decks to wrap up Thomas’ arc since Kira’s promise. It’s possible Locarno will break him out of prison, but he’s already listed as a civilian, which would mean he is no longer in prison when Freeman gets the list. So I think it’s far more likely that Kira negotiated on his behalf to keep her word, and he was released. Whatever the reason, Lower Decks may have a plan in mind to finally close the door on William Riker’s clone.