An Overlord NTR game does sound good. I thought several times about how an NTR story might work; however, this is one of the trickiest ones where normal tropes and setup will not work. The alternative is to use some kind of deus ex machina plot device to make the NTR story work, but that will make the story less plausible and overall harder to play around with ideas and setups.
Eminence in Shadow is definitely a more approachable universe to set up an NTR story for a game. The original plot can be adapted a lot easier than Overlord. The FMCs are hot and numerous, but the MC does not see them necessary romantically, making the setup of an NTR story to need to be more creative in order to compensate for the one sided relationship between Shadow and the seven shades.
One personal choice in terms of an NTR story that can work nicely is Kaguya-sama: Love Is War. It has most of the elements of a good NTR setup. There is romantic interest between MC and FMC, plenty of side characters for extra scenes to fill in the H contents, while the main NTR story is developing. While also having room for extra plot lines or side content for different kinks.
