Well random versus scripted mechanics is a slippery slope kind of issue. When dealing with scripted if you pay attention, no matter how long it takes eventually you will complete the task. The trouble is there has to be some sort of draw back within this system so you get instant death mechanics and enrage timers. This is to provide a challenge while still keeping the fight within the realm of possible completion. Of course it becomes tedious and very boring after being done time after time.
Random on the other hand is another animal entirely. While offering a far more "fun" experience for most because of the "don't know what's next" factor that in and of itself is its largest drawback when it comes to completion. Random mechanics have a tendency of not only surprising you but royally screwing you over and quickly at that. Bahamut's AOE randomized for example would send healers into fits. Can you imagine that popping three or four times in a row?
The problem is not that the game wouldn't be fun with random mechanics, it's more to the tune that because of the way it has been designed surviving overpowering random mechanics like what was stated in the example above is next to if not impossible with the current way things are. Tanks HP pools are a joke along with everyone else's not to mention healers MP. So repeated hits from something of that caliber is going to get tensions rising fast. That means mistakes and a lot of them. For random mechanics to work on here based on how hard bosses hit to HP to MP ratio both pools would have to be nearly doubled or the mechanics themselves would have to be nerfed before randomization took effect. Maybe even both. So if something that hit for 3500 a lick before was randomized and you got bad luck and the tank was hit three to four times by it what then? Cool downs sitting on two to seven minute timers will only get you so far and since it's random you don't know when to pop them.
TL;DR I am all for fresh new stuff but doing something like random mechanics isn't that simple. This game isn't designed for it and before it is jobs, stats and gear are going to need SERIOUS overhauling.