I like this game, here some ideas of mine for the dorp of girls
1) Every time you fight a level boss you increases the chance of obtain a girl, like 0.01%, this porcentage stack until you get a girl, and dont go away if you fight other boss, is independent in every level. somethin like: "every time that the girls of the enemy harem see you in action are more willing to go with you"
2) Other option is a random drop like a item, in this case a pasive item, (like the green orbs) every time you get one, you have 0,5 % more chance to obtain a girl with that boss, all the quest item of the level (like the green orbs) That you did not use become this item at the end of the level (o when you obtain the level girl), all this item is consume after you get a girl, so you have to star again if you want more.
3) boosters (maybe weak ones) must be for a number of fight, no for a amount of time, for people who do not have so much time to play, and must be a booster to girls more easy. (I personally do not see much use of booster right now)
4) if the firts idea if implemented or the second one, can affect the drop in pachinko as well?, is up to you this one.
hope you like the ideas, sorry for my english.