Shapes: Allow more shapes when placing down foundation, Triangles, Circles, ...
or, optionally, allow players to place down foundation frames, which is one side of the foundation, which can then be filled in with material after creating a full loop to create a foundation of the chosen shape
Color: Allow players to use the color gun to change a single foundation, or all foundation for more artistic freedom
Materials: Add more materials that can be used to fill in Foundation Frames as suggested above.