Can you add a side activity that involves more than four players working cooperatively? Right now we have tournaments that accommodate around 16 players, but that is PvP. We also have House minigames, but those are still individually competitive.
I'm not requesting raid bosses or anything like that (at least for the main storyline), since one of the best features of this game is that you only need a group to defeat end game content, but something that brings together more than four players working together to defeat/overcome.
Possibly something like a special battle in Monstrodome that involves 8 players.
An "escape the room" type dungeon where all players need to work together to solve puzzles in order to leave the dungeon.