Open world bosses already exist in the form of hunts. This worked better in FFXI because they could put a cap on the amount of people who could attack it at once. In FFXIV anyone and everyone can attack these "world bosses" so they melt.

For hunts to work properly in FFXIV, the difficulty would have to scale with the amount of people attacking the boss. Same thing would have to apply for an open world dungeon.