I wish they'd just do away with current Ranks completely... They're utterly meaningless as is... I'd much rather PvP Rank be an indication of skill and dedication, than solely a grind like it currently is...
IMO, scoring a Kill should get you PvP EXP, scoring an Assist (which should be a thing already) should get you PvP EXP, winning a match should get you PvP EXP. Coming second? Nothing. Last? That should lose you PvP EXP. Dying should lose you PvP EXP. Perhaps even just not PvPing should slowly lose you PvP EXP, mostly because I want people who grind their PvP Rank for glamours then never queue again to suck it. Getting the Rank would still provide you the Title, but if you want to keep the trophy (the gear), you have to keep playing to maintain it.
Then PvP Rank means something. If you're Rank 50, it means you're winning more than you're losing. It means you're killing/assisting more than you're dying. It means you're a good player. It means if you want to get Rank 50, you have to "git gud", rather than just grinding however many loses...
Give a clear indication of a players PvP Rank next to their name, and suddenly we've got something similar to Battle High/Fever in Seal Rock. Having Rank 50 means you're a high priority target, making having Rank 50 all the more sweeter.
Throw out the AP system, because PvP Skills were honestly never robust enough to justify it anyway (Ninja maybe had potential...) and it just makes things unbalanced, plus I'm honestly tired of it after switching Jobs so much... Perhaps replace it with more of a handicap system, so a Rank 1 gets a bonus, but a Rank 50 doesn't need a bonus.
As for Assists... If you're attacking someone, and someone else gets the Kill, that's an Assist. If you're healing/supporting someone, and they get a Kill, that's an Assist. In Slaughter and Seize, X Assists = 1 Kill towards Battle High/Fever. This should also make playing Healer slightly more rewarding, since I imagine you'd rack up the Assists like crazy, might be some balance issues for DPS/Tanks gaining Assists, but I'm not paid enough (or at all) to concern myself with that.
Then just throw in a "Personal Performance" page to the PvP Profile that keeps track of your overall performance across all matches, all your Kills, Deaths, Assists, HP Healed, Damage Taken, Occupations, etc.. Just do that anyway actually, I really want that...