I am starting a new 4e campaign, expected to last a couple years, and am looking for game mechanic ways to add replacement chars. For example, in our last 3.5 Eberron campaign, if somebody died, new char was added a few levels lower than the rest of the party, but with a number of action points that could only be spent to boost other chars' rolls. For every x action points so spent, the replacement char gained a level, until he reached the party level. I liked the dynamic that this created with new chars, where they were at first not quite as tough as the swaggering old boys of the party, but were immediately very helpful and welcome additions, and had an incentive to help out.
I don't want to do the same thing for 4e, and this is my first 4e campaign, so I would love to hear any clever ideas on bringing new chars into the party. I plan to run a sort of zombie apocalypse game (which is tough in 4e, but we love D&D, and I think we can pull it off), which means one player death every 18 encounters or so, but party integrity is important to us, and I want to have a metagame mechanic that makes it fun to integrate a replacement char into the party. The action point thing was fun, but I'd like something new for 4e.