I had a similar thought about the Denobulans the other day (I'm on an ENT watch through). I guess I missed them in Prodigy.

Back on point, have the Suliban or the Xindi been mentioned at all since ENT?

I was very happy to see the Pakleds come back in Lower Decks.

Would be fun to see what happened to “time machine inventor” Berlinghoff Rasmussen.

I think I’d prefer if they explored strange new worlds more than calling back to existing lore. Then again, what the TNG era did with the Klingons was amazing, and if somebody had made that argument then, we would’ve missed a LOT of quality content.

Maybe it’s possible to dig deeper into some existing species, if done right.

I thought the Miradorn were interesting. The twin bond that they have seems to suggest they have some kind of mental abilities, some way to connect the two people. They were listed as 'quarrelsome', could be an interesting mirror for the Tellarites, coming from a non-Federation species. They sided with the Dominion in the war, too, so they don't seem to think much of the Federation. Could make for an interesting antagonist.

edit: Thought about this some more. What if the twinned Miradorn didn't refer to brothers from the same mother? Perhaps their whole society consists of twinned people. Perhaps they have some sort of process where they bond two of their people together, for life, and that's how they run their world. Think of how dangerous an adversary that would make, there's always two of them. Even better if they have some sort of 2 person hive mind between them, so that they can work together seamlessly, even at a distance. Like some kind of organic grassroots Borg.

Starfleet personnel are a subset of the total human population. Off the top of my head I'm not aware of any canon figure about what percentage of the human population is in Starfleet, or the total number of Starfleet, but presumably it is not a huge percentage of the total.

There are numerous notable civilian characters in Trek, but in general the franchise presents us with a vision where, contrary to the in-universe reality, civilian life is sort of at the periphery of Starfleet. Civilians are people that Starfleet personnel find to be colorful characters, sometimes become romantically involved with, people who serve Starfleet personnel when they're on vacation or otherwise at leisure, sometimes criminals, etc.

The notable civilian characters we see on screen tend to have their entire lives circumscribed by Starfleet -- think Jake Sisko or Keiko O'Brien.

I'd be really curious to see not just a human civilian character but a civilian world fully developed over the course of a series. What is it like to be "just some person" living their life?

As a Relaunch Litverse fan, I really liked the introduction of the competing Typhon Pact as a way to bring in a number of the less profiled species.

Having a different kind of interstellar government from the Federation in a Cold War rivalry for planets and civilizations was an interesting backdrop for the entire era. Creating an opponent that brought together a number of the less known species including the Breen, Tzenkethi, Tholians, Gorn as well as remnants of the Romulan Empire was inspired.

We never got to see as much of the Typhon Pact in the books as I would have liked. The concept though was inspired and one that would fit well in the early 25th century portrayed in Picard.

As for the Sheliak, the novelverse has them in conflict with the Breen for additional complexity.

The newest era of Trek hasn’t hesitated to integrate some of the more successful experiments from the novels, including Una’s name and Illyrian genetic modifications. The Typhon Pact of the Litverse has a lot to offer without being a direct competitor to the world building since Nemesis.

Honestly, the Federation itself. I always thought you could make a really interesting show that was a mix of Federation politics and Starfleet intrigue; maybe a show about the Federation government on Earth that delved into Federation society and dealt with an overarching plot. Like maybe a season where a Federation colony at the edge of Federation space gets into conflict with another race/government, and for a season you get a mix of stories from the colony, a Starfleet ship sent to the colony to support, and the Federation Council on Earth.

Federation politics could be really interesting, but the shows always paid kind of disappointing lip service to it. One key example is in the DS9 episode “Rapture” when Bajor is being accepted into the Federation; the fact that a Starfleet admiral rather than a civilian Federation representative was overseeing the proceeding was lazy to me. Plus the entire ceremony was tiny and anticlimactic (and obviously barely got started since Sisko interrupted). Great potential for some real insight into the government but instead it’s just some rando admiral.

The biggest gap in the existing series is the one-two punch of the Romulan War and the founding of the Federation, which we only missed due to ENT's cancellation. Finding some way back into that era, beyond Riker's holodeck program, would be number one on my wishlist.