I can see several different scenarios that would be in character and interesting. I personally chose to write a Pittsburgh based story in which Melanie and Lindsay returned as well as Justin, because I wanted to write the ensemble. That's what I loved.
I can also see Brian moving to New York after a while, and living there with Justin. It would depend on whether or not Justin liked New York. I can see him hating it. If he loved it, which I think is possible even if a bit unlikely given his personality and how he hated all the pompous artsy friends of Ethan's, then I believe they'd end up there.
I didn't want to write that the first time because I didn't want to lose the big queer family, which is what I loved about QAF. I might, when I'm done with Only Time, write a different post-513 fic in which they end up in New York. I'm not sure, but it floats through my mind a lot.
Melanie and Lindsay breaking up kills me. Brian and Justin raising Gus or any children makes me want to die. I don't need them at Britin, even though I wrote it that way, but I don't think the loft is functional for both of them. So if they are living together in something other than Britin, it needs to be a different loft/apartment whatever. If they are living in seperate cities, I'm fine with Brian living in the loft.
I'm fine with them having a long distance relationship. They're both pretty crabby and might be better off not living together. But eventually I think they would try it again.
I don't have strong feelings about the rings, raw fucking, Drew and Emmett, Ted and Blake. Ben and Michael, who I adore, are OTP and together forever. Hunter is happy and well, also. Those things are quite important to me.
Debbie and Carl are OTP.
Brian keeping Babylon. Hmmm. I don't care too much, but I can't see him letting it fall to the developers, either, so I suspect he'd keep owning it just so he could control its destiny. But it's not important to me personally.
Justin is a genius and a prodigy. He will of course have to struggle to make it, and his success won't be "overnight," but Justin as artist is such a fundamental part of who he is to me that I will not and cannot read fiction in which that's not who he is.
Justin is brilliant. People who know art will always know that when they see his work. I don't think that translates into financial success and solo shows, as I've known some brilliant visual artists who were critically acclaimed and bought by collectors but still didn't have solo shows, at least not in New York or other major art markets, and could barely support themselves.
One last thing: I don't believe Brian and Justin will ever have a wedding, no matter what they do with their rings or monogamy.