I'm assuming that the next update is going to be Skywars due to the recent feedback thread about skywars, but it could easily be a survival games update or a lucky islands update too.
I don't really think that a vanity update would be a great thing to do, as I'm pretty sure that most of the players that aren't on the forums couldn't care less about vanity. Vanity may as well just be added progressively from time to time, but not really worth an entire update in my opinion.
Aside from that, they may just do a misc update, cleaning up minor parts of all kinds of minigames.
I'm hoping that they do a PVP update after (or maybe even before) the Skywars update, as there's plenty of room to improve on & lots of players love that specific gamemode.