What about adding a new cannon sub-type that uses canister shot? Effective splatter to try and take down infantry. But due to reloading you can't get a constant stream of it. So, it's almost like a shotgun for your tank. Deadly against infantry close in, but less effective at longer range. I realize this have very little to do with fixing balance, just a random idea I wanted to throw out there.
It'd be an interesting alternative to vehicle machineguns. Possibly have it occupy the MG slot but fire like a cannon, like you use the MG space as an addon for the main cannon for example.
Or you could just..make it a, "Fragmentation" grenade launcher that..you know..explodes with fragments.
Actually a grenade launcher slot weapon which acts like a shotgun would be interesting, the only problem is that it's on the APC which has lots of anti infantry power anyway.