An idea I had for a campaign based largely on dungeons.
We begin the campaign with X teenagers coming of age on a small chain of islands. The chain is diverse, and the adventure would have guidance for building the islands each kid comes from based on what they want to play, involving asking each player questions in a sort of session 0 mini game. This way all races are on the table, but every species ever printed isn't on these islands. Also you get a group of PCs with ties and bonds in this place, reasons to care what happens to it, etc.
It is the Great Dragon Festival, which is a time to celebrate when the dragons came and saved the world from darkness a thousand years ago, and to light up the night in thanks. The festival happens mostly on the biggest island in the chain, which is home to the Dragon Temple, and so every few years teens from 14 to 16 (small population, there aren't always like 5 15 year olds every year) are tasked with entering the Dragon Temple and lighting the fires inside of it, which will shine through the crystalline eyes of the great dragon's head at the top of the temple.
When the teens gather and enter the temple, they find that darklings have infested it, and even the light crystals that normally glow perpetually have gone dark. It's a fairly short dungeon, but getting the lights back on requires finding the power stations, realigning the mirrors that feed sunlight to the crystals, and defeating the darklings along the way.
At the end of the temple, up in the dragon's head, you get a boss fight with things you have to break or activate or whatever in order to weaken the BBEG. Once that's done, the group is able to light the central crystaline light of the temple, shooting brilliant light out of the dragon's eyes. That done, they are spoken to by the temple itself, which is a sentient celestial construct in the form of a dragon, and thus actually one of the dragons that came to the world to save it a thousand years ago. They're tasked by the dragon with finding the other dragons and awakening them before the forces of darkness do, and warns that if they find one that has been awakened by darkness, it will be corrupted and require cleansing, but once cleansed and awakened, a dragon will protect it's region and bestow a boon.
And then you get a really interesting dynamic where you get a ship, recruit people you know to help crew it, and set sail, exploring a fairly open world but always heading swiftly toward the next Dragon Temple, eventually getting the ship magically upgraded to an airship by the fourth great dragon, and there are 3 of the four Great Dragon Temples already in the sky, and they're corrupted, and they're a huge threat, and one is heading toward the island chain you're from.
The GDTs would be harder and a little more complex, but none of these "dungeons" would be anything like a megadungeon. We're talking a dozen rooms and as many non-room areas, many of which will need to be visited multiple times, as you fight your way through a living puzzle that is trying to help the dark creatures inside and on it fight you off, but is weakened as you cleanse each power crystal and restore it's light, until eventually you have to fight the boss, and you can cleanse the control crystal and return the dragon wholly to the light.
stuff like managing allies, assigning other teams to deal with lesser things or rally allies, forging alliances, etc, would probably be optional, but well supported, with benefits for success and complications for failure. What do you do to help these refugees created by your failure to set up a sufficient defense of this port city that is now on fire? Think Dragon Age Inquisition's war table, with different skillsets available to you for the advisory roles you need.
Maybe end the whole thing with a battle where you get to ride dragons into battle against a big darkness cthulu or whatever, as a cool climax to the campaign, or maybe go the zelda route and you have to make your way into the depths of the cthulu's lair and battle it on it's own ground.
But most of the action happens in dungeons that are dragons.