Not sure if this is already suggested or in the works, but it would be really interesting to see Dungeon instances opening up on occassion maybe after finishing certain large event chains and so forth where you can have many people in one run.
It opens up one of those portals where anyone there can enter within a certain time frame.
Inside would be probably like hell mode dungeons haha normal dungeons increased in difficulty and length. You can run it with a huge zerg but it will still be really tough.
I think that would be really fun and exciting!