I've been toying with this concept for a couple days now.
Game begins with the player party already at a very high level - level 14, 300 character points, six completed careers, what have you. They're epic heroes now: they've conquered kingdoms, vanguished armies, cast mighty spells, inspired songs and sagas with their deeds. They may have adventured together before, or their paths may have crossed in the past (indeed, this may well just be a continuation of a previous campaign), but even if they've never met, they all know each other by reputation.
They're all old now, middle-aged at least, possibly older. They've grown a bit weary. But it's time for one last adventure: a quest for something important that they all desire. For that quest, they meet.
But very little of what they face on this quest is anything new to them: it's not one of those epic once-in-a-lifetime quests at all, it's mostly just same old, undead and dungeons and conspiracies and what have you. They've all been through this shit before. They've fought through worse over the years. It teaches them nothing they don't already know, it doesn't inspire them to any greater heights than where they already are, it doesn't help them grow stronger. Instead, old wounds stack up, fatigue sticks to them worse than in the decades past, and cynicism worms its way into their hearts. They're too old for this shit.
So they don't level up on this adventure - instead, they level DOWN. By each combat encounter, instead of gaining experience, they lose it.
Gradually, bit by bit, they're broken down. They come back down on early levels, and less still, in mechanical terms - but instead of bright-eyed youths, they're tired old men who just want to lie down and rest. Maybe they still manage what they came here for. But after that, they sure as hell aren't going anywhere else again.
Good idea? Bad idea? What kind of a quest could it be?