Yes, there should be something like an "endgame content" for people who have finished all adventure currently available.
But something that would also be awesome, and simple to implement, is an option to invest energy into restocking the store instead of kobans.
Currently i'm coming online only to collect money and buy gifts for the girls, and this would be less boring if i at least could buy enough gifts to do one girl upgrade per day instead of every 4 or 5 days as it is now, as long there is no other way to dump all that energy in something interesting.