I have an issue with character death. I don't like it. This isn't always true, but most games seem to gloss right over "how to keep the game fun for players with dead characters".
For party games, I was thinking of death as something that could only happen when everyone dies, that way the game ends, and no one is left out. The idea is, when a character dies, they miraculously survive. In D&D terms, they stay at zero hit points; they're out of the fight, but they can be patched up/healed afterwards. However, when the number of deaths equals the number of members in the party, that death creates the TPK.
In a five member party, that means five deaths. I was thinking any combination would do it. So, that could mean each member dies once, or it could mean member one dies four times, member two dies once, and the other members never die.
As to how to bring in the TPK, I think that would depend a lot on context and what the players want. Preferably something cool, but I figure it would be narrated rather than rolled. So if you die to being overrun by a Goblin army, you don't play out that battle, you either go out Angel season five style (the character's last words to each other before they face certain doom and then fade to black), or you simply do it in flashes (you know like those scenes in movies where everyone dies and they show flashes in slow motion as the good guys get cut down (like Buffy and friends in The Wish)).
EDIT: A math error.