what if the demoman had a grenade launcher with plenty of ammo and a fast reload time that did less damage- something like a mini-grenade launcher, or an artillery bomber
but the catch is that it has an actually-rather-large splash and causes
teammates, but not you, to do more damage on repeated blows. more minigrenades hitting one enemy causes them to take more and more damage from attacks until ultimately it hits a 2x multiplier:
even more then a minicrit
for the sake of being able to easily identify targets, we could call it a smoke grenade launcher: targets that are suffering from the effect could simply emit smoke, and after reaching the 2x damage cap they begin to emit sparks. on a purely aesthetic level, the grenades all burst into little puffs of smoke as well
no he doesnt
ENGINEER NEEDS STUFF
the engineer needs a gun that lets him 'lock on' to things
like, it could fundamentally be just like any other shotgun- maybe with an extremely low ammo count, only one round per reload- whose real power lies in it's ability to cause his sentry to 'lock onto' targets. when he hits something with this gun, his sentry automatically attempts to switch target (and, if it fails, it breifly searches before returning to normal sentry behavior, which could cause an issue if he's in the next room or something) and begins to score minicrits on this one foe until the engineer chooses to swap to another enemy
(if the engineer is killed with this shotgun out, the sentry briefly 'circuits' and is stunned for a few moments as well. perfect timing for a good one-two from a demoman)
but the catch is that the shotgun itself is rather unwieldly, and pretty much useless while the engineer's away from his post, but by using it a single engineer could feasibly co-operate with teammates to downplay weaknesses that the sentry innately has: like it's tendency to lock onto the enemy closest to it.
basically it's good at fighting bonk scouts and it's good at fighting teams but it's sort of overkill versus one target but if you kill
the engineer the sentry is basically boned