I like the Idea of a 1 or 2 AP-Damage for a Shield-Bash a lot. This would make the ability more useful and also makes sense in the lore. You pushing someone back so he loses some balance and needs a bit of time to get ready again.
Another Idea could be to change the actual Shield-Bash Perk from the Perk-Tree so that you get the AP-Damage with this perk. This way this rather useless perk (at the moment) could become a good an viable option to choose (to make a Positioning- and Debuff-Char)