Sure, but arguably the moment the question was asked by Nikki and the option was included, about whether or not you could be in love with Nikki and in a serious relationship with her, it created an expectation that fans were obviously going to want to pursue.
From that point on, offering an ending that was solo for everyone else accept Nikki would naturally generate some backlash, especially considering that she is the only one with so much influence over MC. It would make sense that MC be angry with her still, but it wouldn't make sense that he remains that way, especially after losing Haley permanently to douch bag, (another scene that in my opinion breaks Haley's character. I'm not saying she should forgive MC and help him to the nurses station still, but it's hard to swallow the idea she would stay with that douch after that) and being unable to return to be with mom. Basically why if Nikki was his only opportunity for love with any of them, would he say no because if rather be alone than with you. That point kind of kills the Nikki Karen ending and the Harem ending all that says is Nikki isn't forgiven which based on other endings obviously isn't true.
Alternatively an ending with Nikki and Haley could work out as well. Could start similarly to a Nikki only ending up to the point where Nikki finds out about Haley walking away with Douch bag after MC gets beat up by him and personally kicks him in the balls slaps Haley and asks her if that is what she really wants or prefers over her and MC. Then offers Haley a triangle relationship with the two of them that Haley accepts.
Alternatively you could start Karen's ending normally until a few weeks later where Haley shows up at your home crying about everything that she missed and how wrong she was about was walking away with Douch bag. While MC struggles to figure out how he feels and prepares to turn Haley away hurt in his eyes because a part of him still loves her and knows better than anyone what it's like to make a mistake, Karen overhears everything and can't bring herself to Deny Haley a part of MCs life, after all it was Nikki who fucked things up, pulling the puppet strings poor Haley was just another victim. The reality of the legal situation also begins to weigh heavily as well as the thought of a future with no grandchildren. Just as MC is ready to send Haley away not willing to risk losing Karen again with a broken face filled with tears, Karen invites Haley in were they all talk about the future.
Yeah just a few bits I thought would kind of fit and would be cool.
