Something that has bothered me for some time is that the sphere entity does not create a proper physics shape when the sphere becomes a spheroid. This is very easy to do in Bullet. Instead of defining the sphere as btSphereShape, use blMultiSphereShape but just one spheroid. As it says in the docs:
" Also since the spheres can be independently scaled in each of the 3 dimensions, a btMultiSphereShape containing just one sphere can be useful to create ellipsoids (squashed spheres), which is not possible with btSphereShape."
This small change would make the spheroid entity highly useful.