I'd start with making PvP "fun".

I know fun is subjective and all, but...all those people who played the feast back when it first came out, when there were insta-queues for both 4vs4 and 8vs8 ultimately quit because it wasn't fun to them. If these people had enjoyed it, they would still queue frequently (though admittedly not quite as frequently) and not just grab their rewards and run away as fast as they can. Unlike dungeons, PvP has a high innate replayability, because no game is exactly the same - part of the reason why DotA could become a raging success with only a single map and no rewards whatsoever during the WC3 days.

I'm honestly amazed that people still think this can just easily be fixed with rewards. If people only queue for rewards, they are gone the moment they get them and they won't even bother in the first place if the reward seems out of reach (Hey there, rewards for the top 100!). And that's assuming the reward is exclusive, if it it's not, people might rather get it another way instead.

IMO, PvP core gameplay needs to change for it to become and stay somewhat popular. How? I don't know. But one thing is for sure: The current gameplay does not appeal to a great many players and that's why all of SEs attempts to enliven it are short-lived.