This could explain why Holly eventually takes up pole-dancing whether you want her to or not. Cuts down a bunch of work making two bodies for every scene showing some skin. I think (if I remember right) that this railroading ticked
BlandChili & potentially some other players off. This would at the very least explain the decision if not alleviate the annoyance.
Besides the visual content it might also just have been done to avoid having to account for the difference in terms of narrative work-load. I speculated as much when it happened too, since the non-pole dancing route didn't really have a lot of unique material, a dinner with Louise but otherwise Holly was just missing from certain Ivy-related scenes, so my guess is that Eva just gave up wanting to continue to account for that option.
After Mark, Robert and possibly Ian, who else do you think slutty Holly will have sex with in the future? Marcel and Billy have both shown some interest in the past, but what about Jeremy or Mike or even Jack? Does Stan stand a chance or possibly Clark?
I think non-changed Holly might do Stan. Changed Holly already gets with Clark under certain circumstances so there is no need to speculate there.
As for anyone in the future, I wouldn't discount anyone except for perhaps Perry. Jeremy could be an option if he gets with either Ivy or Lena, we've seen with Ian recommending Emma to him that he doesn't shy away from sex just because he's been blind to the person as a potential option. Billy and Marcel, like you mention, seem like obvious picks, especially if they get involved with either Ivy or Lena first.
Lena and Holly can both get with Mark and keep their romance intact so if Eva feels like expanding on that I don't see why they couldn't explore moresomes together.
I'd be rather appreciative if she could get with another woman too. I know Ivy is a cock-advocate, but it'd be fun to see slutty Holly getting it on with a woman who isn't Lena or Ivy too.