An idea I had recently was similar to Algolei's, but instead, the characters start off as 1st level. They are told by the priests of the church they are in that they are great heroes that have been brought back to stop a great evil that has resurfaced since the last time the heroes defeated it.
The thing is, the PCs have no idea who they are or who these heroes are. It's like they've been reincarnated, but as completely different characters.
So, you make up a high level group the same size as the current one, and then randomly determine which new character is the reincarnation of which high level character. Now they have to go adventuring to work against this evil and eventually rise high enough in level to defeat it again... clues would be revealed as time went on as to who the different heroes were and what role they played in the defeat of the great evil.
I didn't have a lot of details worked out, but that's the bare-bones of it.