I had been thinking that monk needs some more abilities borrowed from other games to add more fun with flashy spells. I would probably suggest Dolphin Blow (Single Target FF7 and FF8), Flame Dance (AoE FF6 Blitz), Air Blade (AoE FF6 Blitz), and maybe Phantom Rush (Single Target FF6 Blitz also known as Bum Rush). I would have said Aura Beam from FF6 Blitz, but it's kind of in game already as Elixir Field. As for how these spells can be applied, it can be up for debate if it should be oGCD, GCD, a replacement for some old spells, or even an input command with weaponskills. Since the expansion is already out, it is likely easier to just have them use Chakra and offer ways to get Chakra from the rotation such as using a Couerl move with 3 GL stacks up (as suggested by Grimoire-M).

For the Fist Stances, they would probably just work better as cooldowns or baked into other existing cooldowns / traits. Fist of Earth I have only used soloing and Fist of Wind when the boss has their "chicken intermission". It's a wonder these are still in game since Fists of Fire is the useful one "95%" of the time.

For Tornado Kick, I have only used it on enemies about to die from one hit. One way to buff it is to make it do a really heavy burst attack, but that might jack up some PvP matches. It should probably provide a self buff that speeds up the acquisition of GL stacks or reduce the cooldown of Perfect Balance based on how many stacks it "eats up".

For One Ilm Punch and Arms of the Destroyer, they definitely need a rework. The stun and silence is too short and needing a specific stance makes them unreliable anyhow. I am thinking they might as well be our AoE choices for Opopo and Raptor stance with a buff to around 70 - 100 potency to make them worth it for 4+ targets. To compensate, the stun and silence can be removed. This might even be the big chance to put in ideas like Flame Dance and Air Blade as replacements if time permits.