It definitely has to be something exciting, new and refreshing, with some kind of accessible format. What popularized the battle royales was simplicity to get in, and then even higher cross platform simplicity. A fad of that size was last seen with survival crafting games in the early 10's. Whatever it is, I predict it will have a high degree of multiplayer capacity, probably cheap and accessible with microtransations, and easy to spin off. Not sure what that could be.