Hm. Homages tend to be written more in the style of using original characters and scenarios (i.e. SK's 'Jerusalem's Lot' from Night Shift, which iirc was a homage to Lovecraft or somebody like that). Fanfic merely remains 'in-universe'; only the scenario is (somewhat*) original. So while SK was/is a fan of Lovecraft's writing, making 'Jerusalem's Lot' a work of fiction by a fan and therefore fanfic is one sense, the fact that it's a piece of fiction using SK's own/original characters, locations and scenarios sends it over the line and into homage.
(* no fanfic scenario is entirely original, since while the main plot may be the author's own invention, it will be informed by - at the very least, and will most likely strongly feature elements and through-lines from - the work it's based upon; there is simply no ignoring the broader 'in-universe' elements)
With regard to your idea of basing characters on Stu and Fran, setting them in a post-apoc world...well, there's nothing stopping you from doing that. Just call them Steve and Fiona, change certain effects of the disease, their location, and never, ever mention 'Captain Trips', 'Randall Flagg', events in Boulder, etc (in other words, carrying over no elements or through-lines from The Stand - including Stu and Fran's personalities), bring in your own conflicts, threats and resolutions and you're golden. It's pretty much what happened with Fifty Shades, after all. As I'm sure everyone in the world knows by now, that started out as a Twilight fanfic. But even then it wouldn't truly be a homage unless you consciously wrote it in a style that was close to/reminiscent of Stephen's.
sorry
. King creates cherished characters and unique worlds, in reality. No one should try to replicate them unless they have his blessing and legal permission.
