Things I would like to see, or if they're already implemented, things I would like to know how to do!
1) Shared layers. They would be like event sheets where you can define a layer (and what's on it) once and then put it as many layouts as you want. This would be enormously useful for things like game menus or other common overlays.
2) True inheritance. Families are good start, but we can't make subfamilies.
4) Support for vector-based sprites such as lines, splines, circles, etc.