An expansion on the Art Style tab. It seems rather limited as of right now and the three choices don't offer much of a change. Being able to adjust the thickness of the outlines through more than just presets would be able to create more unique looks. Also, being able to choose between dynamic color outlines and uniform color would be nice. What I mean by that is dynamic color is what you have now in which the outline color is based off the color of what it is outlining. Having the ability to choose a uniform would be nice, as in outlining everything in black.
Custom files. As of right now, I don't see that it's possible to make your own assets to import into the software. Adding something like that would be amazing. It would also open up a modding and asset creation community which is good for any piece of software.