A small UI for FAN could be added, in which the Mach number would be set, and the flow physics would change with the Mach number. Also, shockwaves in the flow would be quite a nice addition. This could be used for much better rocket and jet engines (which I love).
Seeing as there is no velocity/speed measurement happening, or at least no UI for it, I don't see a way for there to be a mach in TPT.