Level cap for OBT is 350, however the maps go up to 472, which you can’t really access, so we can assume the level cap will be eventually raised to something beyond 472.
I don’t think they’ll add more maps, maybe way into the future, but they can certainly increase the level of maps (previously it went to 600).
They can add more raids (currently there are none, 1 light raid will be added soon). Dungeons, items, etc.
They can add new forms of PvP, guild pvp, some form of progressive pvp (territory, castle, etc).
They can also add new classes, or hidden classes, or entirely new base classes that lead into new classes, or even a rebirth system. There’s plenty they could do.
I’m sure they’ll probably think of something out of the box, as they have been.