1. The abilities to merge and divide shapes (especially free form/polys) via both the builtin gui and through the object model.
2 . Toolbar customization, at least though the object model. Certain users are dangerous with certain tools and work better with fixed version of those tools.
3. Add a property to Shapes(Index).Line in the object model called .Visible just like with .Fill, etc.
4. Make the size of a shape available as a property. Show and Hide isn't really enough if you need the value for a calculation.
5. Some features, available only through the object model, could be pretty useful in the builtin user interface. Like naming shapes.