How about adding a new trait to shield weapons that deals some amount of damage to enemies pushed back by shields. Does not need to be super high, maybe enough to kill a clanrat in 2-4 pushes?
Or apply off-balance to man-sized enemies pushed (5 seconds of 20% more power vs them). might be op though, what if implement this as passive/active ability or talent (with cooldowns/durations or w/e).
Or include a short duration stun (1-2sec) to man sized enemies pushed by shield users. Would give a tank career a bit more crowd control (since its their job to hold back the tide of enemies and create openings for the attackers) without having to buff/change the weapons themselves. Could even include it as a passive ability for the tank careers (shield specialization).