Brent Spiner's return as Data in Star Trek: Picard was an SDCC surprise no one saw coming, mostly because (2002 spoiler alert!) Data died in the film Star Trek: Nemesis. And while being dead on Star Trek has never been a huge impediment, and in fact, for many characters — from Spock to Dr. Culber — coming back from the dead is a sci-fi rite of passage, resurrecting Data is a bit more tricky.
Even though he is a constructed android, Trek canon has made a big deal out of the idea that Data was fairly unique. In the TNG episode "A Measure of a Man," one guy in Starfleet wanted to mass-produce more versions of Data but was stopped because it would violate Data's basic rights as a person. Data isn't a one-of-a-kind android, but instead a three-of-a-kind android. Called "Soong-type androids" (named for Data's creator, Dr. Noonien Soong, and also all played by Brent Spiner), these three bots are: Data, his evil brother Lore, and the prototype model B-4. By the end of Nemesis, in the year 2379, the only one of these androids still "alive" functioning is B-4. (Data also had an android daughter named Lal and an android mother named Juliana, but they both died, in the episodes "The Offspring" and "Inheritance," respectively.)
Picard is set in 2399, 20 years after Data's death in Nemesis, and according to Picard's voiceover in the trailer, Data is still dead. Speaking after Comic-Con, Brent Spiner clarified that the android parts we see in the drawer in the trailer are pieces of Data's duplicate B-4, but that Spiner is playing Data specifically on the series. "[Data] isn’t in every episode," Spiner said. "He makes some appearances. But he is in the show in other ways."
So what does that mean? Data wasn't just deactivated in Nemesis, he was blown up along with an entire spaceship in order to save Captain Picard. What are the "other ways" Data can come back in Star Trek: Picard? Here are seven plausible theories, all based on the kinds of things Star Trek has done before, or things the show might do based on evidence in the trailer.
Data is a figment of Picard's imagination
Based on the one and only scene in the trailer where Picard is playing cards with Data, it seems unlikely any of this Data stuff will be explained as flashbacks. But, what if Data's "return" is a technicality? What if the version of Data we see on the show only exists in Picard's mind?
This would make the "new" Data a little like when Number Six in Battlestar Galactica only appears in Baltar's mind, sort of like a waking dream. If Data is an aspect of Picard's psyche, the series may never have to actually explain why he's on the show at all. Plus, series producer Alex Kurtzman has said numerous times that Picard would be a "psychological" series. Making Data be in Picard's head certainly qualifies as psychological.
Data is a hologram
This is a popular theory and for good reason. If Data doesn't appear in every episode, it could be because he hasn't physically returned in the year 2399, but still exists as a piece of programming. Throughout Star Trek: Voyager, it was established that the A.I. brains of certain sentient holograms were just as advanced, if not more advanced than those of androids. (The holographic Doctor on Voyager had his own "Measure of a Man" moment in the episode "Author, Author.") The point is, by Picard's time it seems more than feasible that Data's program could have been transferred from B-4's positronic brain after Nemesis and into a holodeck character. If so, the Data we see in the trailer is a hologram with a dead android's body, which, for Star Trek, would be kind of new.
This would also neatly explain why Data calls Jean-Luc "Captain" in the trailer, even though we know Picard is retired and more recently held the rank of Admiral. If the Data hologram is programmed from Data's 2379 memories, he thinks Picard is still the captain of the Enterprise. (Data also existed as multiple holograms of himself in the non-canon Star Trek comic Star Trek: Waypoint, also set after the events of Nemesis.)
Finally, both Picard and Data loved the holodeck on The Next Generation, so it would be a nice way to reunite them.
Data is from an alternate univesre
It's pretty unlikely that the version of Data we see in Picard is from the evil Mirror Universe, but what about alternate alternate dimensions? Though TOS, DS9, and Discovery have made the Mirror Universe the most famous of Trek alternate realities, several other parallel universes have been explored in Trek canon. It's conceivable that a still-living Data could have been plucked from one of those alternate dimensions and now lives in "our" universe.
In the Voyager episode "Deadlock," an alternate version of Harry Kim (Garret Wang) actually takes the place of the regular Harry Kim for the rest of the series. And, more prominently, Philippa Georgiou (Michelle Yeoh) died in the Prime Universe, but her Mirror Universe counterpart crossed over in Discovery Season 1 and now, tells most people that she is the Starfleet version of Georgiou. So, alternate universe versions of major characters replacing their dead counterparts have huge precedence in Trek canon.
Data is a newly constructed Data
This one seems fairly straightforward. In the trailer, we see Picard and the new character Dr. Agnes Jurati (Alison Pill) looking at a drawer full of pieces of an android that looks like Data, though Brent Spiner is now saying that's probably B-4. Still, the character of Agnes Jurati has been described as a "researcher" and if she's helping Picard look at pieces of an android, it stands to reason she works in robotics or cybernetics. Could Picard have recruited Jurati to help him build a new body for Data? Because Data's memories are stored inside of B-4, it seems like it would be pretty easy for Jurati to get those memories into a new positronic brain. Throughout all of TNG, having a positronic brain that didn't break all the time was really hard, so perhaps the reason why Data won't appear in every episode is that his new brain and new body are super glitchy.
Data is a Borg duplicate
The Borg have a huge role in the new Picard series, and though the Borg are not robots, they do have a lot in common with Data. First, the Borg's hivemind is, in some ways, a giant interlinked version of Data's mind, containing the memories and experiences of several people. Though everyone tends to forget this, Data has the memories of 411 colonists from Omicron Theta, stored into his brain, not to mention the memories of his father, his daughter, and heck, probably his cat. The point is, Data's mind is more of a composite of other minds, which makes him similar to the Borg, only on a smaller scale.
The Borg also took a huge interest in Data in the movie Star Trek: First Contact and the Queen (Alice Krige) even offered Data human skin grafts in order to keep him a part of the collective. During this process, it seems likely the Borg downloaded some of Data's memories and program, even in part. And, if Picard is rescuing and liberating former Borg drones in the new series, perhaps a duplicate of Data was discovered on a Borg ship.
Data has been reborn in B-4's body, just like in the comics
So, at the end of Nemesis, Data is blown up, and the only member of his family left is B-4, who is kind of like the android version of Buster Bluth from Arrested Development. In other words, he's not the robot genius of the family. If Data is like the iPhone X, B-4 is like a broken answering machine from 1985.
Still, Data did implant his basic memories into B-4 before he was destroyed (a classic robot-who-knows-they-are going-to-die-move) which left the door open for another Next Gen movie to bring back Data's memories n B-4's body. And now, If Data is alive in 2399, even though we saw him blown to bits in 2379, then what we're seeing has a very obvious answer telegraphed out at the end of Nemesis: Data's memories were reborn in B-4 and B-4 just became a new Data.
Back in 2009, right before the release of the Star Trek 2009 reboot film, IDW ran a comic book miniseries called Countdown which depicted what happened in the 24th century for Old Spock, prior to coming back in time and accidentally creating the Kelvin Universe. In this future, Picard was the Federation ambassador to the planet Vulcan and Data was the captain of the Enterprise NCC-1701-F. The comic book story explained that Data's memories did surface in B-4 and he came back to life even better than before. Interestingly, Countdown's story was created by Trek 2009 screenwriters Roberto Orci and... Alex Kurtzman. Obviously, Kurtzman is now the producer of all things Star Trek on CBS. Would he have revived an aspect of backstory he created ten years ago for Picard? Maybe.
Data didn't really get blown up in Nemesis
The logistics of Data's "death" in Nemesis could reveal an answer as to how he came back to life. Here's the set-up: It's 2379, Picard and Data have infiltrated Shinzon's (Tom Hardy) battleship, the Scimitar, and are trying to prevent a thalaron generator from unleashing a ton of radiation onto Earth. There are only seconds left, and Data throws an emergency transporter beacon onto Picard, then turns around and shoots the thalaron generator with his own phaser, which causes the ship to explode, saves the day, and presumably, sends Data's android body spinning into a million little pieces in space. But, just like the famous death of Sherlock Holmes, we don't ever see a body. Could Data have found a forcefield at the last second that prevented him from being destroyed? Could some freak of physics have saved Data from being killed altogether? Has he just been floating around in space for twenty years?
If Data simply did not die in the explosion in Nemesis and has been missing and in a robot-coma, then literally everything about his return would be explained very quickly on Star Trek: Picard. It would also prove, once and for all, that Data is the toughest and most badass artificial life form in all of science fiction. No matter what the universe throws at him, Data always endures.