Has the idea of customer created controls been considered?
I see that symbols are used to add new reusable UI controls but (it looks like) these can’t be as flexible as the built-in UI controls. For example, the popovers at Mockups To Go look good, but can’t readily be resized or changed without ungrouping and messing around with the individual parts.
In UI Library > iOS, there’s a popover that can have the point positioned in various ways. But if I was after a bit more customisation I would be back to creating controls (symbols) from scratch.