I'm sorry to say this, but a good ending like the MC keeping the love of his girlfriend(s) is not really possible in a good NTR game. I mean there are some well-written netorase games as well, but they are all about sharing, and the Sod2's NTR was definitely better as long as it was about cheating. But when the devs suddenly turned towards the netorase, the story lost its appeal. My favorite heroine, Kirara had the most disappointing final event. A typical poor swinging-threesome, which was more like a pointless fuckfest than a real NTR event.
I think the best way for a sequel would have been if the heroines admit to the MC that they have a lover. Or the MC gets suspicious and starts following them to find out what's going on. Then the story, and the corruption build-up could have continued from here.