i think something that may be cool along those lines is sort of a steampunk looking suit that replaces sentry gun. your movement speed is slowed WAY down (ala heavy) but when you fire your gun is similar to a sentry gun that you have to aim (think sniper scope at close range). he should also have a limited amount of ammo and therefor be tied to dispensers and metal sources.
basically something that makes it possible for an engy to play a bigger function on attack rounds, but also keep him on the back lines because of slow speed and limited ammo. Sort of like the role demo plays in competition play.
another idea is a replacement for the wrench that heals at a significantly lower rate, but has a minicrit, crit or stun chance when it strikes a firing sentry gun (strike the gun and a stunning baseball flies out of it). or maybe it heals at same rate but when hitting sentry gun, YOU take the damage that the sentry gun is taking. maybe also give it the ability to disrupt spy's disguise or cloak ability? (similar to what sniper shield does for backstab).
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