I don't want to tell you how to make your game but what if you made it so that getting a harem is one of the hardest things to do so that people can do a challenge runes? where you have to balance character stats, work around their traits, max out your own stats, and learn even more ancient and forbidden spells. while not being found out by the school or any government body and maybe becoming an evil villain or using your profound knowledge to defeat him. honestly, that last part might be Fecher Creap but yay I think it would be cool if doing something like having a harem is one of the hardest endings to get, would be really fun and rewarding too.