An idea I wish to share: Since we're harshly limited by slots and there's so many types of girls around... what if, when we release a max affection girl into the wild, the game saves her, and exploring the appropriate area has a chance to have a past girl drop by, with mostly filled hunger and sleep (she's doing well for herself!), full of coins you can pet out of her, and ready to be released again.
Maybe even have like a "guest slot" that cannot be used normally and is separate from the normal one (so a expedition brings you an egg AND might have a old favorite drop by if the slot is free)