TNG gave us the Ferengi, Borg, Sheliak, and Cardassians, among others. And they gave the Klingons more depth.
If we were to introduce additional new baddies in these later seasons…
Actually, a few aliens I really would like to revisit are the Conspiracy Parasites, Nagilum, and the Schisms aliens. They could all be interesting in different ways.
The Parasites could be a bit like the Changelings in that you don’t know who’s friend or foe (they could disguise themselves further next time), and they could be taking over different people from different powers. You go to thwart a Parasitic takeover of New Romulus and the Klingon ship at your side turns on you. Plus, what is Parasitic culture like? Are there good guy parasites? What’s their homeworld like? Endless seas of zombie hosts? Are there different kinds of Parasites we have yet to see? What does eventual peace and normalcy look like with them — them crawling around trying not to get stepped on, or do they merge with hosts and walk among us?
Nagilum was one alien. What are the rest of his people like? Or his arch rival — “The Master” to his Doctor? How else might they toy with our heroes and learn about our lifeforms? What are they like? All we have is a face and know that they’re very different from us — they don’t die.
Are the Schisms experiments a prelude to extra-dimensional invasion? Are wormholes going to start appearing and fleets of soldier aliens spewing out over key worlds? What are their people like?
…beyond them though, as we’re returning to the original mission and pushing outward into unknown space (the original mission for the Galaxy Class starship was to be away for 20 years, ergo the families and environments onboard), maybe we meet aliens who are a different shade of cybernetic. Ones that seem to be dancing on the razor’s edge of Borg-dom. I’ll call them the
Vesuvians for the time being. They have AI’s that smoothly run their world industries and governments, and they plug their implants into their Metaverse-like realm in cyberspace, but now they’re also starting the earliest neural Links with each other. Artificial mind-melds. Can people Link in small groups then really choose to return to “reality” as more limited beings? Or once they get a taste of it, are they hooked like addicts? How benevolent are the AI’s running the hardware, and can they influence the Links? Or can extremist technocrats/oligarchs in their governments/economies — for the greater good, surely. This could well be us by the end of the century.
Visually, I’d see them as paler but very similar to us, with smoother features overall and more Transhumanist tendencies toward playing with their appearance to look sexier and to stand out. They color their hair, tattoo and pierce their bodies (both with great care to compliment their forms), and they play with their genome, adding animal (or Trill) spots, some playing with animal parts in decadent displays — not far off from where we might go in the age of cosmetic genetic-engineering. Their cybernetic implants are ergonomic and jewelry-like; that is when they’re not ironically ugly or anachronistically steampunky.