Self propelled Artillery.
It have to cost 5 IPC and move 2 spaces, that is a given.
Attack and defend on a 2 or less, for simplicity, and not to unbalance the Tank.
I suggest it should be able to boost two infantry units, because of better mobility than the stationary field artillery.
But now, we also need to boost the Tank, or nobody will buy it.
Tanks, cost 6, move 2, att 3, def 3 and blitz, but can also absorb one hit when attacking, as in the A&A 1914 OOB rules.
The new Assault gun Tank destroyer unit is just a Tank with no turret, cheaper to build, cost 4 and move 2. Weak in attack, since it has no turret, but strong in defense since its low profile. I say attack 2, and defend on 3 or less, and boost one matching infantry only when defending. This units special ability will be to, either target attacking Tanks, since that is the reason they made it, but it can only target enemy Tanks when defending, since it will be a defansive unit. It will be kind of like the Anti Air gun that target aircrafts.
Or, if we agree to let attacking Tanks absorb one hit, the Tank destroyer can negate that ability.