I'd like to see an ability to summon the gatling repeater from haunted halls as, say, a 1/rest or maybe a 10min cooldown ability for artificers when they get around to adding a 3rd tree for them (maybe a more trapper & mechanic centered tree?) and give the ability to the rogue mechanic tree too to help make them more viable in epic levels.
Maybe a similar ability to summon the big ballistas could be good too.. you'd probably have to balance it by adding a set number of bolts (4 or 5 probably) as ammo into the inventory of the summoner that are used up as you fire it, meaning it'd have to be manned by them & would prevent it from being used all the time.
Sneaking in & laying down the big bomb is already a laugh for rogues, and who wouldn't want to be in a party where a viable option for taking out an opponent is to break out a siege engine?