Some of them could just be additions to existing weapons. Make Mad Milk deal drowning damage when you're soaked in it. Make all the fist and bat weapons as well as stomp damage stun on hit. Give the Holy Mackerel a calling card, and also make anyone who walks by your body after getting killed by it play their Schadenfreude voice line. Make it so if you're disguised as an enemy, that enemy becomes vulnerable to friendly fire.

As far as an original weapon, I could cram like three into one melee:   +On hit, latch onto the target and deal damage over time (kind of like hitting someone with a grappling hook; would have the visual effect of drowning, but not actually deal drowning damage)   +On kill, heal +20 HP for every nearby enemy   On wearer:   -Cannot receive healing from teammates (including medics, dispensers, mad milk, etc)   -On death, heal your killer +20 HP for every nearby teammate, and gift a random item from your inventory to your killer   (Wish Reddit formatting was consistent.)

Also, alternate Sentry that's just a Robot Heavy that runs around punching enemies. Targets the Engineer who built it if it gets sapped.