Slice2D "levels" (editor can be used to construct a collection of Slice2D instances which can be loaded at runtime).
Slice3D "levels" (same concept as Slice2D levels, but completely different editor, format, etc. and of course a different runtime element).
Slice2D tile-based levels for platformers, etc. to easily 'paint' levels, and automatically avoid weird collision bugs due to adjacent corners etc. as can happen in Box2D (the editor can compile single tiles into large tiled squares to get around this problem, and use similar logic for visual tiles to streamline rendering).
Possible alternative to Slice2D+Box2D for the tile-based levels+systems -- a completely separate physics engine that is designed around tiles, and a separate rendering system that swaps textures in and out of atlases in order to draw all visible tiles in a single draw call. Dynamic elements smaller or larger than tiles can be simulated, such as players and enemies smoothly walking along the level, or destruction particles that bounce off the ground, bullets that bounce off walls, etc.
Incorporate the Sword language for scripts to be written in all of these editors, allowing interactions to be programmed even by untrusted mod developers without the danger of compromising users' machines.