I feel like asteroids would be a really cool thing to put into the game. For one thing, it just allows us to do more stuff. Recreating some iconic real life missions like OSIRIS-REx would be awesome. Another thing is that it brings more difficult challenges to the table like asteroid hopping! Hopefully these can have non 0 inclination orbit so that the solar system doesn't feel so flat. Sorry if these were already suggested but I feel like they would be so cool and challenging to have in the game.
Also: Moons of other planets ;)
There are actually already bodies in the system that have non 0 inclination, tydos for example and cylero aswell, its just very small. Also if you wait some more time untill the planet builder comes out you'll be able to make an asteroid yourself.