Just like...
you know...
i hate to say this...
MW2.
BUT ONLY because that's the only example I know of.
A player type that slowly regenerates health after a few seconds of not being attacked. Perhaps change in speed of recovery through preferences.
Instead of the screen going red on receiving damage, it would get harder to see the more hurt you are, and once you start recovering, it gets clearer.
Would this be possible?
I just get tired of waiting until the screen clears up, then forgetting about my health and getting killed by a single bullet a few minutes later.