How I'd build a new-school-friendly Open world dungeon crawl.

Dungeon is completely open world with multiple paths and rooms criss crossing in a labyrnthine design.
Bosses placed behind "Instance doors" and "Door guards".
For the party to face the boss/named mob, they have to work their way to the room. Clear the Door guards, and then zone into the instance with the boss fight. Once the boss is killed, the character cannot attempt the fight again for X minutes (mimicking the old respawn timers). Once the fight is completed, the group is returned to the Door of the instance and can crawl to the next boss on the list.

End result, all the dungeon crawling goodness of the old school without the "spawn camping". All the new-school interesting mechanics without the complete community disconnect.