Remember those bosses in FF history that absolutely destroyed you because they had horrid status effects? Normally you could handle huge damage, even if it cost a phoenix down... but things like Confuse or Berserk? That threw everything off kilter.
I'd personally like to see a return of classical status ailments. I realize some of these are already in, but they're boring and uninspired.
Confuse
What it does: Makes you run around hitting friends, foes or yourself
how to cure it: Getting physically hit / Esuna / Remedy (thrown)
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Mini
What it does: Turns you into a miniature version of yourself, significantly weakening you, and lowering your maximum HP and halving magic power
how to cure it: Esuna / Remedy (thrown)
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Zombie
What it does: Either reverses healing into damage (I could see how this could be used to grief though) or makes it so you can't be healed at all for the duration
how to cure it: Viruna / Esuna / Remedy (thrown) / Holy Water (thrown)
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Charm
What it does: Forces you to attack your friends and allies
how to cure it: KO (Maybe something else, but this should be a nasty status ailment)
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Doom
What it does: Activates a countdown timer, when it hits 0 you die
how to cure it: Holy water (Low chance of removal) / Killing an enemy (100% chance of removal)
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Disease
What it does: Halves your maximum HP, MP and all stats, does not fill HP or MP on recovery
how to cure it: Esuna(?) / Viruna
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Berserk
What it does: Makes you unable to control your actions, but sharply increases attack / speed(?). You will not be able to choose a target, but you can only target enemies.
how to cure it: Esuna / Remedy(?)
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Toad
What it does: Turns you into a Toad, limiting you to greatly weakened physical attacks
how to cure it: Maiden's Kiss (?)
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Amnesia
What it does: Can't use weapon skills or job abilities
how to cure it: Esuna
These are just a few ideas, I want to actually fear varied status ailments and be in situations where multiple ones used at once would be very scary. Currently the status ailments are pretty potent, but they're generally pretty boring imo.