Think twice about killing

This mechanic is quite a general idea and could be applied to different types of games so long as they have killing in it.

The idea is that there is a maximum amount of killed enemies. Killing something when the maximum amount has been reached will respawn the strongest of the currently enemies to make room for the newly killed one.

This makes you think twice about whether you want to kill the small mob in front of you or just evade/spar with it becuase you may cause a boss to respawn by killing it