This can actually happen in this game. For example in Final Fantasy tactics, there was a stat, Bravery and the higher that stat the more damage and defense and what not you would get. But if your bravery was ever low, I believe 0 or 1, you would literally turn into a chicken. Now we have Determination, maybe if a boss puts a debuff that starts draining your determination if it isn't removed and your determination reaches 0, then your character would just sit down or go to sleep on the floor or something.

Other things that could be added would be turning you into a Chocobo.

Personally I would like to see a boss that has all sort of Mental powers and that it would affect how the game looks. Instead of charming you it makes all the players targetable and makes them look like him, so even though you are not charmed and can choose to do nothing, the boss can be healed or you can damage your allies. Another thign would be to turn your character into a very pixelated or blocky ps1 or ps2 graphics version of yourself.