Shield bash should do stun damage imo, to represent the opponent falling back and having to recompose themselves for another attack. Alternatively a defence penalty for 1-2 turns (or to expire on the second action basically) as they have to rebalance their defensive positioning.
If that is too overpowered then having a failed shield bash cause you to get the defensive penalty might be a negative to ensure that people don’t go out bashing willy-nilly, it could represent a shield bash being withstood, causing the ‘basher’ to fall back / be disoriented / lose balance and require them to adjust. Pointedly, I think it would also need to cancel any pre-applied shield wall.
I think that would be a viable, if risky method of breaking down that last guardian or pest that you’ve surrounded but won’t let his defences down whilst being extremely punishing is misused at the wrong time.
