I've previously thought about who the girls might end up with if MC doesn't choose them (because let's face it, they are not all going to a nunnery for the rest of their lives after you reject them). I don't think you're going to get the kind of NTR that you're looking for, however. It will just be an epilogue where DPC describes what happens to each of the main characters after the events of BaDIK.
The #1 clear choice for Jill is Rusty, now that he is "changed." They're a good match and it just makes sense. The only other person on campus who might have a shot with her is Jacob because he's an artsy guy, seems mature, and doesn't act like a DIK 24/7.
For Sage, she might go back to Chad; although, that seems less likely because he's got one foot out of the closet at this point. I see her landing a successful guy in the corporate world whom she meets after graduation.
It's difficult to imagine Bella having a non-MC good ending (or good, MC ending for that matter), but if she doesn't fall to pieces, I think she'll end up with an older guy who is not currently involved in the B&R story. Maybe she meets him in therapy?
Maya & Josy will obviously end up together if MC doesn't choose them. And I don't think they will select a surrogate MC to fill in the void.
This a college game and people are hooking up all of the time, so who knows who might get together in the future, but I just don't see DPC putting in hardcore NTR, so you may be out of luck in that department.