I have so many ideas. This program is wonderful as-is but there are so many things that could make it that much better, in my opinion.
The ability to import your own models would be fantastic.
I also found myself wishing that I could have a map grid that properly overlayed itself close to the terrain (or painted directly atop it) so you can use the scenes as 3D maps for tabletop games. I know there's an existing map grid for orthographic view, but the ability to create 3D gridded maps would be icing on the cake.
The ability to export a dynamic image, gif, or video of your terrain would also be absolutely wonderful, or even just being able to get rid of the HUD so you could use a third party screen capturing program to get a clean video of your entire camera view would be enough on this point, honestly.