I think that would be an overall good approach to start with. There are many possibilities how to tweak it. If it was to stay the way it is, I think gradually increasing the amount gained by the kind of target would be a nice idea. You could also bring other things into the mix, like headshots healing temp health, headshot kills healing more, critical kills healing more, consecutive attacks against the same target within a certain time frame without hitting other targets, killing or damaging specials and/or elites, “saving” teammates, reviving teammates, there are a bunch of possible key actions that could warrant temp health.