I see, overall it's a game in which the MC will seduce girls and gain dominance over them. Overall from a storyline point of view it's kind of really generic, perhaps adding something extra could make the game feel more unique.
I will just say a few random combinations (with succubus an lust as the elements) from easy to hard, perhaps some of them may be useful:
-Light 1: In the game we will meet a playful succubus that wanna have fun and have chosen us randomly to help her, she will offer her help to make you get the girls to submit, overall having sex with girls generates lust power and that makes the succubus powers stronger to help you in your adventure.
However in the last part of the game you will notice that the succubus help isn't free, but rather she is the one in control of the girls so the MC will try to take her over to finally get his harem.
-Light 2: Same as light 1 but the succubus wants to make you submit to her powers too.
-Medium: Similar to light 1 but the succubus objective is just to play around with you, her objective is to make you become submissive to females and the more you use her powers, the more she is actually affecting your character's preferences.
-Hard: The succubus just wants to make the school into a fuckfest, increasing the lust of the students, she didn't care about your wish but rather found you as an useful tool for her objective.
Overall these are just concepts that try to add a level of challenge or that the storyline isn't so simple. You are perfectly fine by doing the generic concept because it can work, however it can be added something else to make it more special, the concepts are just a few ideas to show how you can change a lot a game depending on the use you make out of the game factors.
