On the note of Zombie themed worlds, it might prove to be a bit difficult to do without encroaching on Guro territory - which to my understanding doesn't have a very large fan-base with the general hentai gamer populace. I could perhaps see zombie tentacles being a thing though, similar to the cult-bros in Resident Evil 4. If you do decide to go with the Zombie route, another possible series to use in there would be "Kore wa Zombie desu ka?" Looking forward to seeing everyone's ideas on the worlds though!

In a bit of a different direction, have you considered doing a full Succubus/Incubus themed world? Think something like Astarotte no Omocha. I think it would fit with To-Love-Ru and Highschool DxD, as well as some other series that weren't as mainstream.