Heres a idea to the chain reaction:
If you backstab the heavy being healed by a medic for example, the heavy would die, his corpse would leave a spark effect (similar when you kill a medic with full ubercharge) and the medic would suffer slowdown for 2 seconds with half of his health

Also: if you drawn your knife underwater, you would suffer an health decay like when you are in afterburn until you draw another weapon or get out of the water

What do you guys think? Sorry for bad punctuation or mistakes.