The main advantage of wave springs compared to coil springs is that you can have the same stiffness and travel for less uncompressed length. This stems from the fact that a compression type coil spring needs to have an open helix to provide room for the spring to compress.
In comparison the various types of torsion springs don't change their dimensions much as they are loaded and so the advantages of a wave spring don't really apply. In fact their relatively compact size is one of their main advantages in a lot of applications.
What would constitute an improve design depends very much on exactly what the constraints of the system as a whole are.
There are a few types which fall broadly under the category of torsion spring, including solid bars, helical springs (as in mouse traps) and bundles of fibres.
There is a long history of using bundles of fibres as torsion springs in catapults dating back at least to the roman balista and medieval mangonel