Or how about, instead of using battle points, AFTER you clear a world, the boss uses energy. That way, those who have cleared all the worlds to date, can still use their energy and not feel like it's going to waste waiting for a new world/mission to open. Or if someone still needs girls from any of the worlds they cleared, they can try it with energy instead of battle points. You could even make the cost like 15 energy, since we get way more energy than battle points.