While not a strict rotation we were due for another Assault map as every other type had already had two maps released since vanilla OW (List of maps by release date - Overwatch Wiki)
That said I desperately wish they’d create new game modes and more maps/heroes per year. I think 4 would be very doable and they’d still be able to retain their level of polish. I even made a post the other day about new games modes I’d thought up. (shameless self plug below)