Make some nozzle styles for the solid motors like the Echo style to be able to vector their thrust. The parts are already here you'd just need the extra code work and some animations for each nozzle. With this we'd have more realistic SRBs and proper ICBMs or other rockets that use Solid Motors and fuels.
Oh also to avoid having them be used over their liquid fuel counterparts, they can only gimbal up to something like 10° or even 5°
@Frolonov Maybe those are another nozzle styles