Star Trek: Discovery Season 3 takes place 930 years after Season 2. That means, in theory, the crew shouldn’t encounter anyone from Trek canon who wasn’t already on the ship or a fellow time-traveler. The year 3187 is so far past most Trek canon, that Discovery Season 3 is essentially a sequel to all of Star Trek up until this point. Everyone involved with the show, including star Sonequa Martin-Green, has pointed out this time-travel jump will give the new season a “clean slate.”
Does that we won’t see anyone from the expansive Trek universe? Could legacy characters who aren’t time travelers or space gods (like “Q”) still cross over to the new Discovery? One familiar face from Deep Space Nine could easily return for Discovery Season 3, and there are already clues from the trailers that it will happen.
Based on the 2019 New York Comic-Con trailer for Discovery Season 3, we already know several classic alien races will return this season, like the Andorians and the Trill. The Andorians aren’t immortal or long-lived (as far as we know) but the Trill certainly are. In two scenes in the trailer, we clearly see members of the Trill, and we also see Burnham in what appear to be caves on the Trill homeworld. We also see a new character who appears to be floating in the Trill pools.
To refresh your memory, here’s the trailer again.
Below are screenshots of the relevant moments.