Another approach is not a single dungeon, but a complex that includes multiple dungeons. For example, a necropolis that includes multiple tombs, each of which is its own dungeon. Each delve is a mission to a specific dungeon within the complex ... but if the players want to wander about and enter another one, they're welcome too.
This is actually the design rationale of my Jakalla Underworld megadungeon in my Empire of the Petal Throne (1975) game. Each level is a series of distinct areas. On level one they have found a sprawling series of catacombs (unnamed but containing individually-named areas, such as the Watch House of Eternal Vigilance, where they had to fight a bunch of zombies), a sprawling undercity of abandoned basements and storerooms, a small but functioning temple (The Black Abode of Putresence Triumphant; actually it's not functioning for the moment because they slew the priests), a tomb complex (the tomb of Kalvar the Cruel), etc.
The tomb complex they explored actually had a pit which they did not explore... it in fact leads down to another tomb holding a vastly desirable prize, but they were gun shy about spelunking it after the shoggoth in the tomb's treasure room ate one of the party (the rest did well to escape).
In the whole campaign, which has lasted a few months, they've only explored less than half of level 1... of course the dungeon was designed to be approached in multiple forays and for some areas to be "missed". There are also multiple entrances and ways to move up and down in the dungeon. Conceivably, we could have multiple parties exploring the dungeon, but so far I only have 1 group for it (down to 7 players right now, but that's not a bad number).