As far as the Romulans go, I really like the concept of the Qowat Milat. Also the expansion of Romulan culture to the point where we learned a surprising amount about their gods was neat.

I think Jurati's Borg have some interesting implications, and I'm very curious to know what the deal with that transwarp conduit at the end of season two of PIC is. Of course, we're never going to get a real follow-up on that.

And that's just from PIC, a series that I think is hands down the worst overall series of Trek.

I think the mycelial network is extremely interesting, and wish Disco would do more with some of the implications introduced in season one. The 10-C from season four are also very intriguing.

LDecks has been surprising about expanding on the cultures of various species, like the Pakleds, and the Orions. It also introduced the idea of the support fleet. And while I don't generally like it when death is something of a revolving door, I do think the idea of the Black Mountain is more interesting than the extra-galactic synths.

The thing is, we don't know much of anything about the EGS, right? They left the admonition as a warning to other synthetics, and when they show up, they're a bunch of metal tentacle arms. There's not really much to go on there. I would like for it to be expanded upon to, for lack of a better term, justify the inclusion in Trek lore, but when they showed up, I can't say I was anything other than disappointed.