Just checking since some source material leans towards different tags then others. I'd probably also pick something that either hasn't been used in a while or even hasn't been used at all for games at least for a source.
It's hard for me to decide between Zero no Tsukaima and Kanojo, Okarishimasu. On the one hand, a fantasy world where you can add combat and scenes related to it, but I've heard more than once that people don't need combat in such games. Also, due to the fantasy, it was possible to add some magic to the antagonist with which he will put a mark of lust on Louise, which according to the plot will develop and force the heroine to become more and more corrupt. On the other hand, the world of Kanojo, Okarishimasu is very suitable for the NTR genre, but there is nowhere to do combat and the story will revolve around the fact that the antagonist was either previously rejected by Chizuru or accidentally found out about who she works for and thereby blackmails her, gradually raising the bar. Perhaps Mami will even help him (of course, in her own interests).
