How exactly won't these shapes enhance the experience of a fight? Designing a raid fight is obviously a creative endeavour, and it is in the creators best interest to utilize as many tools at their disposal as possible which includes the environments. For exceedingly large foes a doughnut shaped arena might work where the boss is stationed in the middle and there are mechanics forcing the players to move clockwise/counterclockwise, alternatively the boss can stand on the arena and chase us along the doughnut. For triangle/cross shaped arenas each corner can be dedicated to it's own mechanic or change up the bosses behaviour when it is inside of them, or the boss teleports around where we have to traverse an obstacle course to reach it. With elevated platforms, the arena can be split into two parts where half the raid need to manage different mechanics. An arena idea could be a rotting/burning tree and as the fight goes on we have to climb it to avoid the rising rot/fire.
Of course all of these things can be done in a perfectly square or circular region such as P2 and elevation, but it should be in the developers best interest to spice things up and to be creative.
Also by now people should have learned not to use their movement abilities, gap-closers included indiscriminately because jumps aren't jumps. If there is a pool of fire and you "jump" through it you will take damage.