Give Weather Effects in the game actual usage rather than being cosmetic

[Kaliesto]

I can see this happening in the open world, but in instances?

Best case, numbers are tuned slightly or it just becomes another mechanic for the boss/dungeon.
Worst case (like in your Garuda example), people will only bring jobs that benefit/don't suffer from the weather, making FOTM worse.

Will just need to account for Job's little tricks, and have different exclusive penalties.

Like disabling Dragoon's Backflip during Garuda Extreme because the force winds will cause it to be unusable or alternatively the Backflip if used will give the heavy status.

and Black Mage's little teleport thingy will cause some damage for going against the aetheric wind at high teleport speed.