What I would love is to be able to use modern laser mice with vintage PCs -- I have a Mac Plus which is a nightmare to use because those old ball mice are flippin' terrible.
I think you could fake the quadrature signals pretty easily. When I designed my laser mouse, I initially wanted to be able to reprogram the microcontroller to output USB, PS/2, quadrature, etc. but that got dropped so as to just get it out the door... but I think a USB->quadrature adapter would be pretty doable.