I think the terrain painting tool is very primitive so I have a suggestion that would make a huge improvement.
LayerInfo files could store a max height and max slope variable, which when you select the layer and paint over your terrain, it is only painted according to those variables, this will allow designers to quickly paint sea beds using max height, paint cliffs using max slope etc.
When I think about this further, what would be really great is… if we can create custom paint tool blueprints, so basically… you go to terrain paint mode, and able to select a custom “PaintBrush” blueprint, inside these blueprints would be a construction script that can configure different alpha shapes, powers, angle limits/constraints. This would really allow people to increase their productivity with 'scaping and painting.