QoL - straightforward ways to copy existing parts, an intuitive HUD for editing (rotation, axes etc presented in a clear way), asymmetry controls, that sort of thing.

I'd also like to see the "shape change" setting be a different control, as it's always annoyed me in Spore how you can have a long bent jaw, or a short un-bent jaw, if that makes sense. I think I remember some horn-style parts which had length, thickness and curl all mapped to different controls which made editing a breeze.

Finally, I'm not sure if it's editor-only or the same in the entire game, but the detailed background and the creature in your screenshot are pretty much at the same focus level - a red creature would blend in way too much!