Balance, Homogenisation and Encounter Design: The Problem.

[Shurrikhan]

I want to tell you a bit about a game that released just over 6 years ago. It had the things you are asking for but due to various reasons things did not work out like you would hope they would.

The game had a fair number of uniquely shaped boss arenas with varied terrain. In fact the game's most difficult fight on release was set on the palm of a gigantic dragon's hand. These irregular arenas while cool had their fair share of problems; things like ranged classes being able to avoid a mechanic because they could stand on top of a pile of sand and players failing to dodge a fight's mechanics because the irregular terrain prevented them from moving away fast enough. From the second update cycle and on the designers ended up changing to flat arenas to prevent these problems.

How is this a flaw of terrain rather than a Z-axis in proximity checks? Or is the fact that even now I can be struck from 40 feet in the air by ground mobs still aggro'ed to me similarly an issue of (open world) zone design?

Kiting used to be a thing in some of the high end fights, but due to their only being one ranged non-caster at the time that class became a mandatory class to complete content.

Only T1 and T5 had kiteable adds (unless stall-stratting T4 briefly instead of doing full AoE), and I did the prior just fine on SMN and BLM with very little potency loss and the latter was doable on any and all jobs, seeing as you could just keep hitting the boss while everyone else dealt with the add focusing you. These weren't an issue.

Homogenization came about because the designers needed to insure that any balanced party composition had all the tools they needed to complete content.

The one does not necessitate the other. You can have balance without homogeneity.

The designers were originally doing what you asked but problems and complaints caused them to slowly stop doing them over the course of their first expansion cycle.

Problems and complaints should not to equate in each case to giving up and taking the lowest common denominator approach to the situation.