If champions were to die permently after receiving their fifth (or fourth, or even third,) wound, this would have a couple of effects on game play. For one thing wounds would be more meaningful, the player would have to be more careful with an injured champion, and the injuries would be much more meaningful.
It would also lead to serious consideration of the idea of retiring a champion to a city to manage the city, as he/she couldn't be risked in the field anymore. It would also reduce the crazy number of traits that some champions get. I find that even in a relatively short game, some of my champions get this really long list of traits that starts to feel a bit much.
Just a thought.