The things I want most:
Criminal gameplay overhaul, from the mechanics to better punishments (risk and reward etc etc), with lots of attention given to smuggling (I think smuggling fits the game-world perfectly and the existing mechanics are very well suited to making great smuggling gameplay with some tweaks) and of course piracy gameplay.
NPC comms; I was surprised back when I started in 2015 that we couldn't communicate with NPCs even in basic ways (beyond combat). Demanding cargo, requesting help, asking about POIs, receiving missions, tricking them, etc etc, most which we could do in Elite 2. Even if it was a very basic system, I think it'd add a lot to the core gameplay since so much of Elite is a single player experience.
More missions, a variety of challenges and rewards that make sense, classic space-simmy things like reconnaissance, escort, blockade running, perhaps exploration missions where a faction asks you to find 3 undiscovered terraformables for them to use as options for future projects, etc etc.
BUT, I do think Space Legs could be way rad and I'd be very excited if they can pull it off. And I'm fortunate that I'm one of those VR players who doesn't get motion sick from walking around, so I hope the experience is great. It could add a lot, and even though I'd be fine if the existing core gameplay was improved in a variety of ways and we never received Space Legs, I'm pretty pumped if Space Legs is coming.
EDIT: Oh, and I'd kill to be able to save loadouts that we can switch between with a few clicks of a button. The outfitting screen is so dang slow and I've spent perhaps billions just in spare ships decked out for different jobs to avoid the outfitting screen. But I'm easy!