If they attached to the Character characters, progress the game. No need to fear epic progression, as you could sway the game more into role play rather than roll play. Are any of them married? Run any guilds/factions? People look up to them? Set them up to make bigger names in the world, live, love, train n00b5.
Another option is to simply put them up against something they can't handle. And when they die their signature weapon become a legay weapon for new PCs to seek out and stop new threats with.
Another option is to consider the PC's to have so much power that they become godlike, or become some elemental or spirit, have them create new PCs that are avatars of their old characters.
Or just straight up kill them 19 times.