Grease Lightning isn't even the main thing i'd rather see changed, another person said they rather Monk keeps the position limits they have but other then one or two moves on Dragoon they are the only class that requires you to be in a set spot. And if the run calls for you to move a lot your damage is literally cut in half.
If anything else just removing this option would be good. Of course they would need to lower the over all power some to cut the bonus they get for not having to move but to keep up a decent output of damage i'd say that is a fair trade.