I personally hope for a path where normal Ian and slutty Holly finally become a dedicated couple.
Unless you're specifically talking about a Holly that has had sex with Mark, Robert, Clark, and more, then entering a dedicated relationship with Ian a "slutty" Holly and normal Ian can already get together. Holly's change has to do with how much Lena, and Ian to a lesser extent, encourage Holly to be more outgoing and get out of her comfort zone. It's entirely possible for Holly to smoke weed on the beach trip and wear the skimpier bikini and still be with Ian.
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?
Clark can already be one of Holly's conquests when Ivy is showing off to Lena during Lena's little intermission this last update. Robert, Mark, and Clark can all have selfies sent to Ivy that Ivy will then share with Lena. Marcel is certainly still a possibility, I have my doubts about the others you listed. Yes, Billy showed some interest, but he's starting to feel more like Lena's evolution from Stalkfap into porn, at least to me. Mike and Jeremy both seem doubtful, at least for Holly to be getting with, as they both have never met her, though that can easily be remedied, as well as arguing they've met her off screen when the girls have hit the club.