I don't think the new feature will be anything too big. Certainly no ship interiors or earth-like worlds (seriously, it's not possible to make those happen in a way it's realistic... None should resemble another and their features would need to be generated depending on the star type, location... everything. NMS is fantasy space without any realism, that's why it works there.)
For planets I'd be content with fluids, weather and more volcanism and the like. But I think that would already be too much to expect.
The new feature will be something smaller. If there's interiors, maybe it's the cockpits/bridges of our ships. We could finally use that coffee maker in the Krait MK2...
Or maybe we'll be able to own megaships and stations? That would use mechanics already in the game, but put more strain on the server. So maybe not.

Personally I want it to be associated with exploration. There's been combat, Thargoid war and now a Powerplay update, and none of these things were and are for me at all. I'd love this to be an update to the stellar forge adding more features to the nerdily created galaxy we play in. Things outside the bubble. Realistic space phenomena, more variations in stars and so on. Comets, white dwarfs eating their companion stars, acretion discs for black holes for example... Things outside the Bubble.
Buuuut I don't think that's it. Maybe we'll just get an extra slot on our ships for finally fitting spoilers without the use of shipkits... Wouldn't that be something?! ...