Starting in 2017 with Star Trek: Discovery, fans were ushered into the 'Nu Trek' era of Star Trek. It's an era that started with the aforementioned Discovery series and has gone through Star Trek: Picard, Star Trek: Lower Decks, Star Trek: Prodigy, and Star Trek: Strange New Worlds. The era is far from done, but those are the key series that have made up this new era.
During the Nu Trek Era, one of the things that have become commonplace is the reliance (or overreliance) on past characters and actors from previous shows. Every show has either over-relied on previous characters or previous talents to bolster their show. It started with Spock and later Christopher Pike and Una Chin-Riley in Discovery (among others). Lately, we've seen the likes of Chakotay and Harry Kim pop up in Star Trek: Prodigy and Star Trek: Lower Decks' respectively.
In fact, we've seen a large number of characters return across Discovery, Picard, Strange New Worlds, Lower Decks, and Prodigy, with even more expected with the upcoming Star Trek: Starfleet Academy.
Yet, we haven't seen every major character, so who hasn't returned for the new era just yet?
Well, while Strange New Worlds has gone through a good number of characters from the original series (as it's a direct prequel to the first ever Star Trek show), they've yet to bring in Hikaru Sulu, Pavel Chekov, or Leonard "Bones" McCoy. It's impossible to say if we'll see those famed characters in some form or facet before the end of Strange New Worlds, but we certainly hope so.
Picard really did a good job bringing back the crew of The Next Generation, bringing in every major and mid-major character from Jean-Luc Picard all the way down to Wesley Crusher. One name never returned, however, and that was Natasha Yar (aka Tasha Yar). She was briefly mentioned in the third season of Picard, but Yar nor the actress who played her, Denise Crosby, has appeared in any new show since the end of The Next Generation.
The Next Generation isn't the only show to have a large portion of its cast pop up in future shows. Take, for instance, Star Trek: Deep Space Nine.
So far we've seen Worf, Garak, Julian Bashir, Quark, and Kira Nerys all pop up in the new era, with all but Worf appearing in Lower Decks. Worf, of course, returned in Star Trek: Picard. Also, it should be noted that the Bashir and Garak we got in Lower Decks weren't the same characters as the ones from the Deep Space Nine series. Despite that, we're counting them.
That means that the only characters from the Deep Space Nine series we haven't gotten yet are Benjamin Sisko (who has returned in the official comics) and his son Jake as well as the pair that carried the Trill-symbiont named Dax, Jadzia, or Ezri. Lastly, we've yet to get Miles O'Brien, though he at least appeared as a statue in an episode of Lower Decks.
As far as Voyager goes, that show has had a fair amount of its cast pop up. Between Prodigy, Lower Decks, and Picard, every major character but Neelix and Kes have returned, Neelix is always a possibility to return, but it seems unlikely we'd ever get back Kes. Not only does her species only really live seven or so years, thus making it hard to work her into an episode, but the woman who has portrayed her has seemingly left acting after a series of unfortunate personal incidents.
The crew with the most to make up for with regards to appearances is the crew of Star Trek: Enterprise. So far, only T'Pol has appeared in another show, Lower Decks. Like with the Garak and Bashir entries, the T'Pol we got wasn't the official Enterprise T'Pol, but she was once again played by Jolene Blalock, so we're counting it against. That means that everyone else from the show has yet to appear in anything new.
Hopefully, a trend that doesn't last too much longer.