The biggest issue is that they have to get away from the idea of extending gameplay with super hard boss encounters because the target audience for these games has to make use of matchmaking tools far more than they once did to get things done in a reasonable amount of time. Fights like "The Naval (Extreme)" and Twintania are designed to force individual players to wipe repeatedly in order to learn the fight and thus demands that all members of the party be on the same footing if they ever hope to win. The combination of hard boss fights plus duty finder/party finder creates a situation where the chances of success on those encounters are absolutely abysmal in a random group. So the replayability and reward of the game need to originate from someplace very different.