I could picture a minor game mechanic being added where a fraction of all xp that the player receives is also received by their current battle team. It wouldn't be a huge difference, making even the largest xp gains in the game effectively worth less than some of the cheapest xp items for those girls. But it would soften the "wall" of reaching the level cap since you could still be working on leveling up your harem after that. With something like that, xp wouldn't lose all meaning for a player until ALL of their harem reached the cap (and even then there are new girls being released to push that second "wall" further back).

I cannot see them adding a mechanic to bank unused xp for a future change to the level cap though.