So I know that opinions vary on how close this game is coming to becoming XI-2, and there are also varying opinions on if that is a good thing or not.

However, I think XI came across something in the later part of its life that could completely destroy standard things like the 5+ ARC setups we see at moogle fights currently.

That thing is the stagger system. Not a direct port of it, but a variation (eg, the mob becomes invincible until a certain ability is used, which is randomly selected from one of the seven basic classes) could vastly increase the balance of some (It would be stupid to use this system for every fight in the game) future endgame content.

Seeing as there are 8 slots in a party, and 7 combat classes, I don't see no reason why all of the content in this game cant be done with one of each class in the party, I currently believe that for a skilled enough group of players it is possible, but of course it is the player-base's choice to run the builds they do. I think that for the battle devs, it might be a nice idea to occasionally not give them that choice.

As for when the number of classes exceeds 8, the random ability list for a battlefield could be restricted to one or two of the jobs that are favoured less by that battle. (E.g. Howling fist required for the final blow on Ifrit - not a serious suggestion, but an example of how things could work)