I wouldn't mind open world dungeons if they were made and balanced for lets say 100 players. If more entered, they would get into a new instance.
It isn't pure open world dungeon as it would be still separated into small channels, but making one instance for whole server isn't obviously possible anymore without getting pure zergfest. You would still enter the dungeon normally trough map, you would still encounter other parties and will be able to compete or cooperate with them, but you wouldn't have to worry about whole server coming in and spoiling the fun.
If the bosses would be scaled like FATEs and would require multiple FATEs to be done in order to be summoned, it would be kinda nice in my opinion. But OFC the difficulty of these "FATEs" would have to be higher, with some normal fight mechanics etc.