Short Answer: The Eugenics Wars; Deep Space 9 movie
Star Trek is one of the original franchises. It encompasses many series and films, as well as mountains of work in other media. The main focus now seems to be creating new shows.
In a very brief period of time (since 2017), we have been given:
-Picard, which tells us what happened to the former Next Generation captain, um, next...
-Discovery, which first fills in the gap between Enterprise (the series, not the ship) and The Original Series, then (spoilers) leapfrogs past all the other series, chronologically, to show us (finally) the beyond year 3000...
-Strange New Worlds, which connects Discovery and The Original Series, with the adventures of Capt. Pike (whom we met in the Original Series' FIRST pilot episode and rencountered in Discovery)...
Plus, two new animated shows: Lower Decks, a comedy about what happens there, i.e. not on the Bridge; and Prodigy, meant to engage kids raised on anime-style animations. It introduces a whole new crew... but also (spoilers) Janeway from Voyager (from the series and the ship), shows up... sorta.
So, yes, the new series are doing a great job of looking both forward and backward (if one can say that about a show set in the future).
But there are still more gaps to be filled in.
Namely. two: The Eugenics Wars and the rest of the Deep Space Nine story.
The Original Series got movies. The Next Generation series got movies.
But Deep Space Nine never got movies.
All of the actors, save one, are still with us. And the one who passed on-- Rene "Odo" Auberjonois-- was a shapeshifter. He could be "played" by CGI, or by an entirely new actor as long as the resemblance is passing. After all, it is canon that he had a hard time assuming human form. Now that he has been with his "people" a while, it is possible his powers of mimicry have sharpened, which could easily explain his new face.
If ever there was a series that makes the viewer beg to know what came after the series finale, this is one.
Further, the end of the series involves a battle that surely would have drawn the involvement of Starfleet's flagship-- Picard's Enterprise. Yet, what that ship was doing during the battle was never shown... or even discussed! Weird, right?
We see Picard meet Sisko in the pilot of DS9, to pass the torch narratively. So we know these two leaders are contemporaries. We also know that Sisko blames Picard for his wife's death.
What about a movie showing what Picard and his ship were doing during the battle for the planet Bajor and its wormhole-- and have it end with Picard making such a huge sacrifice-- for, say, Sisko's son Jake-- that Sisko has to forgive Picard for his wife's death? Forgiveness and redemption are huge parts of Trek's ideology.
The other plotline that has been referred to endless times, but has never been shown, are the Eugenics Wars. The "augments" that were the result of genetic engineering gave us the enhanced ubermensch named Khan, Trek's best baddie. We see him being exiled in The Original Series. We see him escaping exile and wreaking havoc in the Wrath of Khan movie.
But we never see the origin of the augments, which is a shame because it also involves three characters we know and love. One is Dr. Soong, who eventually gives up on perfecting humans like another, Khan... and turns to AI, resulting in the building of Data. (Khan's full name is Khan Noonien Singh. Data's creator is named Dr. Noonian Soong. These names are, in real life, a tribute to a friend of Gene Roddenberry, the show's creator... but could easily be used to link the two characters.)
The other is a character-- who was inspired by James Bond and The Man From UNCLE and such fare: Gary Seven. The character was created for one episode of The Original Series and was intended to spawn a a spin-off. This show never materialized, but the idea of humans protecting Earth from alien threats (and with the help of a memory-wiping pen, at that!) perhaps gave us Men In Black.
The difference is that the MIB agents were from Earth. Gary Seven's bloodline originated on Earth, but his genetic ancestor was abducted ages ago; he was the result of millennia of genetic perfecting on a distant planet, then beamed back to Earth (in The Original Series) to protect it from threats foreign and domestic... er, alien and human.
Anyway, Gary Seven (whom we need to refer to by his full name because "Seven" to Trekkies means Seven of Nine) had tried to get the augments (this is explained in some Trek novels) to be defenders of Earth, only to have them decide to conquer Earth instead.
Which resulted in, you guessed it, The Eugenics Wars. Which... should have happened by now, IRL. They happen 1992-96. The other major gaps in the timeline, in terms of years not chronicled, lie (still!) between Enterprise and Discovery... and between The Original Series and The Next Generation, even though the movie Generations has characters from both.
It would be great to see more of Gary Seven (name-checked in Picard) and his assistants, a brilliant if awkward Roberta Lincoln (played by Terri Garr, still alive as of this writing) and a shape-shifting black cat Gary Seven can communicate with telepathically. Oh, and Gary Seven knows what Vulcans are-- and is immune to their neck pinch, so Vulcans can show up, too, as long as no one else sees or identifies them.
So, showing us the Eugenics Wars gives us, potentially a story with Dr. Soong (or someone in his ancestry), Khan (whose enhancements evidently allow him to live for centuries; Wrath of Khan is set in 2285), and Gary Seven (who could easily still be alive in the 1990s if he was in he 1960s, especially since he is a "perfected" person)... plus a war that takes place in the Trek timeline in the 1990s...
...before the year Picard's (the show) characters go back to (2024), well before warp-drive tech was developed and "first contact" was made (in 2063), let alone the establishment of Starfleet (2161).
I guess what I don't get is, if Dr. Soong was still perfecting augments in 20204, how could the Eugenics Wars have taken place back in the 1990s? I think I have more research to do...