Playable Naga & Satyr would be neat too, in my opinion. I saw some people doing the upright naga suggestion, and in fact the Nerubians who are upright in this upcoming expansion reminded me of that a bit. Personally I’d hope they’d keep their snake tails, but I know that’d be kinda hard to do with mounts. Armor isn’t a problem though, given the dracthyr.
For San’layn, I think the most obvious choice is the missing blood princes, Theraldis and Atherann, along with using Naxxanar in that temple of En’kilah (spelling?) and/or Ghostlands. All of the above would be really neat to explore.
We still don’t know where Blood Prince Dreven came from. Plus, who knows. He could very well come back. Much as I utterly hate Shadowlands, the beginning of the expansion before everything went to the garbage had been interesting, even if the rewards were wholly lackluster.
Lorewise - Lana’thel, the blood Princes, and some other scourge members were brought back to cause chaos. There was also Lana’thel’s soul or something found in Shadowlands, I don’t remember much about that though, as I didn’t play the expansion through because again, I just really hated it. There were the one-off lore tidbits here and there that could come up again.
Some folks bring up Tha’lena. Who knows, she may be used again. I think exploring her story would be neat.
But by far I am most interested in the missing blood princes, and wouldn’t even mind a new character coming out of the woodwork for the San’layn like Dreven did, though this time without the villain bat and something to explore a viable story that makes sense like DK/DH/Forsaken.