I don’t know if this already came up, but I’d have an idea for an engineer ability. A repair drone. Remote controlled and/or with some kind of lock-on mechanic; airborne, or grounded. That drone could lock on to its target objective, if the engineer is inside a certain maximum range of the repair-objective, and fly, or roll torwards it. Or maybe the drone could be remote controlled; even though I would prefer the fire and forget version.
I don’t want to get too much into the specifics, like how much, or little health this thing should have, its cool-down time, etc., but it’d be cool, if you could also use it as a bomb in some way; some kind of secondary mode, but more as a gimmick and not too effective. For the maps that don’t have an EV the drone could help in other tasks, like “capturing” the switch in Trainyard, etc., you get the idea.