Wouldn't this actually require the players to memorize the ENTIRE move set even more because the system is basically a deck and the players are required to count cards to generally know when it's time to start feeling safe from certain moves PLUS when the deck has been reshuffled?

Personal rambling anecdote here? I'm playing from Asia in the Montreal data center. My connection is bad and I need a vpn. Back when ARR started I didn't have one and starting as a paladin going up against my first end game content Ifrit was very daunting, especially since it was 50/50 that my role was to stun down eruptions. With my skippy latency it was completely impossible to stun eruption with reaction alone. My solution then was to memorize the entirity of Ifrit's move set. After I mastered it, it was obvious that I needed only to memorize the preceding attack pattern before eruption to get to it but back then Ifrit was a big deal for my i50+ self that needed his sword bad. I memorized his move set like it was an exam for me.

Now, ever since getting a vpn that works for me, I've been able to fight bosses while generally knowing the important points of a fight for a specific role and just react to that. For example, I only know how to fight nael as a tank and all I need to remember is 1st stardust, meteor stream, 3 gcds after 2nd meteor stream, etc for raven's beaks. Ask me anything else about nael's phase 1 full move set order and I wouldn't remember and weirdly enough, due to the way the battle system to server interaction works, I think it's a good thing to a degree. It's lenient enough in a way that low latency players can just ignore the whole rotation system and react to moves as they come or play by feel and high latency players do not need as much reaction because they only need to memorize bits of the rotation.

I do realize that technically this game is not marketed/intended towards me and some people will feel that the game shouldn't be held back to cater to my own real world shortcomings so to speak but I've seen people playing from America or somewhere closer than mine and has it worse than I do. Going back to my first paragraph, with your proposed system, the need for twitch reaction will be heightened and they will need to 'count cards' for the whole thing to help them get past hurdles instead of memorizing bullet points. Don't you think that maybe most of them would find that just as unfun as memorizing the static rotations?

Make no mistake though, I realize that most of these request for randomness in boss fights stems from that fun you get from boss fights in single player action games and I enjoy them too. The game has a fight similar to what you (the op) and others want, I think? Stone vigil hard. It's an enjoyable fight that has players approach a fight differently compared to the other boss fights in the game: actively looking at the bosses and reacting to their animations and attack paths. It's like fighting a double rajang or double tigrex except...it's super easy. Their tells are super long and their damage is puny. They're 4 man dungeon boss fight though, so that's acceptable. But what if they're scaled to 8 man trial/coil in terms of damage and reaction time required? Where exactly would the balance needed to be? I've seen this dungeon enough times to realize that the majority of the player base would have trouble if they're only given time to react to these bosses' random moves about as long as they have to react to t1 and t2 high voltage or a chimera dragon/ram. If the damage is lowered to not be as punishing, then it boils down to having the healers play a harder whack-a-mole because people just keep getting swiped left and right for non lethal damage and it's the healer's job to not have them die.

Man this sure is tl;dr...hope someone actually reads it LOL