When enemy HP increases for every member beyond 6 in endgame content, this mandates that every job in the party be absolutely vital, and it severely limits the jobs that can participate. It's not the players' fault that they have to behave in a close-minded way.

The solution (AKA the suggestion that you will never have any plans for) is to only increase enemy HP based on job repetition within an alliance. For example, an alliance with 18 different jobs should receive no HP increases in the monsters. This would create incentive for players to team up in large groups and to welcome a wide variety of jobs. However, a group of 6 that includes 2 monks would have the HP of all enemies increase slightly. A group of 18 that includes a ton of monks or rangers should have the enemies' HP increase a lot.

And that's all I have to say about that.