Purify is infuriatingly useless, and its probably the easiest thing they could change to make it better. Give immunity to further CC after its been used for like 3 seconds, or implement diminished returns.

To fix newbies rushing into ranked headfirst with not even their spells on their bars they could implement a PvP tutorial that you have to complete before you can actually queue into it. But first we need a team of PvP developers who are capable of understanding whats holding FFXIV pvp back, and taking over the direction of PvP moving into 7.0. Because I doubt the people in charge of PvP, assuming there is anyone, know what they are doing. The class balancing will likely be impossible to get 100% even, but the willy nilly nerfs that make literally no sense, and stupid map changes, and lack of incentive to PvP need to go.