The thought is simple. The moment a Boss is killed in HC Zombies, it gives all players in the lobby an (1) Extra Life.
I had suggested this on Discord and Durtle deemed this as something you can suggest on the Forums so it should be possible.
Also I had another thought, since the faster you kill the Boss the more points you'll get, then **if possible** the reward for the Extra Life should be the same as well. Up to three lives. Like if you kill the Boss real quick(For Points it's like 14,500 above) then its an Extra 3 Lives, 14,000 is 2 Lives then anything below is just 1 Life.
Bosses appear in Rounds 5, 7,10 and I forgot about the upper Rounds. It shouldn't be too OP since it'll give you only like 9 Lives maximum for Rounds 10 and below. While this also is more rewarding, I see it as something that'll CHALLENGE the players more ( into killing the Boss just to get more Lives, because really.... buying Life/Lives is/are costly ).
But if what I had thought is a little too much, then please consider implementing the upper part instead.