Nutshell: The Incredibles meets Dynasty in Waterdeep.

Characters:  Everybody is a former high-fantasy Dungeons-and-Dragons adventuring hero; the group hid the upper levels, gained tremendous fame and fortune and eldritch power, and retired to their homeland’s cultural capital.  Mix, match, create your own — these are just examples.

• The Rogue, society patriarch.  In his youth he led the adventuring company (What was the company’s name?) to fame and fortune, before settling down.  Now his combats are in the parlors of high-society matrons and the backrooms of exclusive clubs, everybody smoking big cigars.
• The Fighter, society matriarch.  She never thought she’d settle down, but the Rogue made her an offer she couldn’t refuse.  The Fighter has settled uneasily into the role of mother and noncombatant; she keeps her armor polished and secretly hopes for a massive orc invasion that will pull her back into the field.
• The Wizard, lecturing academic.  She’d like nothing better than to retreat to her own private demiplane and engage full-time in plumbing the Far Realm.  It’s a poorly-kept secret that unlike the rest of the company she never really retired.  She’s assured the Cleric that their children are truly his, even if she decided to grow them in vats rather than bear them in her belly like a savage.
• The Cleric, lovelorn celebrity.  As the highest-ranking member of his god’s church on the continent (if not the entire world) he’s constantly drawn into disputes large and small.  One day the Cleric is requested to arbitrate a border dispute between the orcs and the dwarves, the next he’s judging the pumpkins at the city fair.  The Cleric is too generous to refuse any of the honors and requests for appearances made of him, which keeps him away from his family much of the time.  Just as well; he suspects none of them can stand him.
• The Sorcerer, daughter of the Wizard and the Cleric.  She’s ninety percent certain she’s an ‘improved’ clone of her alleged mother.
• The Bard, son of the Fighter and the Rogue.  He and the Sorcerer are engaged, but that’s purely a political thing; his relationship with the Druid is an open secret.
• The Ranger, half-orcish half-sister of the Bard.  No one ever talks about the Fighter’s affair with the Barbarian, perhaps because the Barbarian’s abandonment of the company was the proximate cause of the group’s dissolution.
• The Monk.  No one ever suspects the Monk.

Setting:  Waterdeep!  City of Splendors!  Or, if you prefer, Sharn!  City of Towers!  Or even Greyhawk, City With a Castle Nearby, or Palanthas, City from Dragonlance!  Regardless of which classic D&D setting you settle on (Which classic D&D setting did you settle on?), pick its largest city and cultural center, the place designated for “urban adventures” back in the 1980s.  The cast — the PCs — are the remnants of the most powerful group of adventurers of their generation, with all the high-end magic items, ridiculous wealth, power disparity, and infighting that implies.  Half the party married the other half and produced a second generation. These children are now adults and heroic adventurers in their own right, but toil under the shadow of their impossibly accomplished parents.

All the standard D&D-isms are accepted truths of the setting.  Pick an edition you prefer (I’d use 3.X or older) and stat the PCs up if you’d like, at whatever constitutes ‘impossibly high-level.’  These character sheets have absolutely no mechanical significance of any kind, but may aid in conceptualizing the PCs.

There are no monsters to fight and few villains to slay; the handful of powers in the world with even a minimal chance of defeating the elderly PCs in a straight-up fight know to stay well, well away from their home. Instead, their greatest enemies are one another.

Possible Themes:

• Non-Lethal Damage — just as hurtful as the real thing.
• Poorly Worded Wishes — magic backfires as often as not.
• Communicates via Grunts and Gestures — telepathy is easy, empathy is hard.
• Many Things, Deck of — no matter how much you stack the odds in your favor there’s always the chance for the unexpected and unpredictable.
• Rod of Seven Parts — doing the impossible merely requires breaking it down into small enough pieces.
• And an endless litany of additional D&D puns and in-jokes.

Tightening the Screws:  Depending on the setting, you might be able to wring a lot of drama out of the Time of Troubles or Spellplague or Second War of the Lance or whatever.

Other concerns involve politics and people, not dragons or dungeons.  An envoy from a distant land, one powerful enough to meaningfully threaten the city if not the PCs, appears and expects to be feted.  The orcs or their local equivalent petition for aid or threaten to invade or both.  The nominal city government falls under the sway of a charismatic stranger.  Someone’s devil, demon, or other outsider comes to visit and/or terrorize.  Economic collapse, perhaps triggered by sudden massive inflation as the market is flooded with gold recovered from deep dungeons.  Wars and the rumors of wars.  Indirect assaults on the city and the PCs’ power base.