Naoki Yoshida interview @Gamescom

[Felis]

So basically, hunts and diadem in a new coat of paint.

Not 100%
There can spawn a portal or a NPC you have to click on to join. It can send you to the instance (if the battle is instanced) or can work like a leve to unlock the boss for you. This way the maximal numbers of participants can have a limit.
And they can build in a gearlevel-synch into that mechanic to keep the boss challenging even after 2 more patches.

Open World content can work, so long it is not open battle with no participants limit.