CSG Painting
Ahhhhhhhh, Consecutive Solid Geometry, that thing games like Restitched and LittleBigPlanet need in order to be even remotely fun to create levels in. CSG could be used in Toolbox for creating a material smearing system like in those games (and will be, some day), but it has other uses as well.
CSG is great at being a way to paint landmasses on a 2D map, erase parts of them to form waterways, and can be combined with a LittleBigPlanet-like corner editing system. You've just created half the functionality I need for Socially Distant's city map.
CSG could also be used for creating an in-engine mask editor tool. Instead of needing to render a circular avatar in Socially Distant using a circle triangulation algorithm like we do now, one can render a single quad with a mask texture containing a circle drawn in-engine with the CSG tool and baked in the game assets. Same end result, less expensive computationally, more avatars visible at once, and you get the flexibility of being able to mask out a hexagon too! Or a weed leaf!