I voted Bolian and Tholian, not just because they rhyme but because they're both races who have been long established as existing but with very little actually known about them.
I would like to see Breen, Tholians, Tzenkethi - Typhon Pact races basically - but only if they use the material already established about them in the books. I know they're not obligated to do that, obviously, but with Kirsten Beyer in the room it's at least a possibility. Of course it's also a potential reason for them explicitly not to use those races - because doing so either ties them to other people's creations or risks alienating fans who want them tied to those already established things.
I dislike when Star Trek constantly invents new alien races with every new iteration of the show. There are enough aliens already established - I want to learn more about them, not get constantly bombarded with new randoms. But I don't want races we've already had a lot of time with either - Bajorans, Klingons, Ferengi, Cardassians. We already know enough about them, let someone else have the spotlight.
I want something in the middle. Bolians are the perfect example - they've been around since the first season of TNG but apart from a couple of tiny titbits we know nothing about them. They like eating aged meat, you can feel their pulse through their ridge, you can't share a bathroom with them, they're okay with suicide. That's all we know. They're just brought out whenever we need an alien in the background to provide some colour, but let's get to know them as a culture, as a society, they way we have with other races.
That's what Discovery is perfectly placed to do IMO - not invent new aliens we've never heard of in the future, but explore already established aliens who could just use a little more attention without tripping on canon.
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