I'm sorta with the shoulder cannons theme. Maybe have the bots convert or transform into additions to the engineer based on the type of bot being converted or transformed, granting one or two abilities from the converted or transformed bot in addition to some type of visual effect. From this, it could either be applied as a short-term cooldown or a permanent application with some sort of drawbacks like not being allowed any bots out and summoning one would cancel the effect. On the other hand, they have a temporary transformation, so it's not like such a pet-powered concept will come into fruition. Ultimately, despite the bot menagerie not necessarily being a mandatory fixture in the engineer's vast arsenal of toys, they are still a convenience. That one distraction from the engineer's attacks, that one source of extra damage available even when the engineer can't get a clear shot, that one snare to let an injure engineer make their escape or hold down the prey for an all-out strike, even that one extra healing source when pots are on cooldown and there's no healer in sight.....it all adds up to a reasonable amount of added efficiency. As annoying as pets and any other character minion systems can be for some (myself included), the sans-robot alternative looks more and more of cutting off your nose to spite your face.