When I'm defending a keep, it feels pretty worthless getting a kill against so random idiot who thought it was a good idea to stand directly in front of the wall. His zerg just has to pile up, and he's back.
If you die a second time within a certain time period, and without respawning on your own, it should require two soul gems to bring you back. A third time? Now your buddy needs three soul gems and has to determine on the spot whether or not you're worth it. Whatever kills we get while protecting/attacking a keep don't really matter until we're actually face to face with no walls between us. I would really like my kills to mean something.
I understand this may ruin the fun for some "duelists", but then again, this was designed for large scale pvp (supposedly).
I don't like the idea of any debuffs as a death penalty, that would just rise the number of excuses on why you died. But making your teammates decide whether or not you're worth rezzing over and over and over again because you can't stay alive, that seems like a good option to me.