I got another request to do a Stancy post from @webuserspam and a Ronance post from an anon so I think I'll just do them all at once. So in this post I'm still going to discuss Jancy endgame, Stancy endgame, single Nancy and Ronance endgame and I'll say my thoughts on which are most likely at the end. Full disclosure, I don't really like Stancy, I like Jancy and I love Ronance but I'm going to try and write them all in a satisfactory way. Just in case I sound biased.
In terms of the time skip, casting websites are asking for cars from 1988 meaning there'd be a time skip of 2 years. This would mean after the first few episodes they'd skip ahead to where the kids would be 16/17 and the teenagers would be 19-21 (I know they're all teens but it's easier to differentiate that way). For me this pretty certainly spells out a Jancy breakup because they will be at separate colleges so Johnathan will have to admit they can't go together which means breakup.
However, this season is described as post apocalyptic so maybe they both decided to stay in Hawkins, although I doubt that. I feel like Hawkins is going to be taken over so the group flees Hawkins but something happens and they all have to come together again to defeat Vecna once and for all. Kinda It Chapter 2 esque, where he's been defeated for now but he's coming back. Either way, I'm going to talk about it as if they do go to college because I don't know why that conflict would exist if they were never going to go anyways.
Ok first of all, Jancy endgame. Early on in the season, Johnathan is going to have to tell Nancy that he isn't going to Emerson with her. They're both pretty awful at communication, especially since they're long distance, so I don't think the relationship will be sustainable. This means pre time skip, I think they'll break up. However upon reuniting, Johnathan and Nancy are working together doing investigative work. It's a bit awkward but amiable. Spending so much time together reminds them that they love(d) each other and as all hope seems lost and it seems like the world is going to end, they turn to each other and rekindle their romance. They know where they went wrong last time and they're prepared to make it work.
Next up, Stancy endgame. Like aforementioned, after realzing they won't be together at college, Johnathan and Nancy break up. Steve will either be at a (community) college or still working. Either way, Robin will probably be there with him which is irrelevant for now. Steve has given up on casual dating and is now trying to commit to someone. Who knows, maybe he even had a serious relationship at some point in these two years. As a result of this, Steve has moved on from Nancy but he knows he's never loved a girl more than he used to love her.
When Nancy comes back to Hawkins, she's been broken up with Johnathan for two years. She's more concerned by the inevitable doom than romantic relationships. Similar to Jancy, Nancy spends more time with Steve and they're reminded of why they were such a good match in the first place. Previously, Nancy couldn't bring herself to love him because of Johnathan, but now she's able to commit to him. When all hope seems lost, she shares a final kiss with Steve but shockingly they survive.
Steve and Nancy talk afterwards and decide they'll keep the relationship long distance because Steve wants Nancy to follow her dreams but it's on an optimistic note. That or one of them dies (Steve most likely) and the remaining person (probably Nancy) is able to say I love you to their grace as a sad tear jerker moment. Especially considering Steve's whole dream for their future that he never got to achieve because he died so young for his actual "nuggets", leaving behind Nancy and the kids.
Next up, single Nancy. This one is a bit more simple than the other options. After breaking up with Johnathan, Nancy throws herself into journalism at college and is doing very well for herself. She drops this to come back to Hawkins. The town is now famous for whatever the hell is going on with it so Nancy claims it's to write a crazy article. Nancy spends time with both Johnathan and Steve but ultimately she tells them that she has to pursue her dreams without romance holding her back and maybe when she's older they could get together, but for now she's got to be alone. This isn't to do with romance, but I think if Nancy is writing an article on Hawkins it could be a big exposé and could be used in voice over to cap off the series. If we're being really cheesy, she could even name drop the series.
Finally, Ronance endgame. When we see a scene of Nancy at college, they could drop a hint about Nancy liking women, like her blushing at someone or something along these lines. Nancy and Robin have been sending each other letters whilst Nancy was at college and their relationship is strong. Robin still isn't out to Nancy and has a whole will they won't they thing going on with Vickie.
When Nancy arrives in Hawkins, their feelings for each other become abundantly clear as they spend time together. Robin wants to tell Nancy that's she's a lesbian but she doesn't want to damage their friendship because she's scared Nancy won't be as close with her if she comes out. Robin is smoother with Vickie than she was but the minute she's with Nancy all of that smoothness goes out of the window and there's this kind of flirtatious awkwardness that hangs over the relationship. Eventually, Vickie asks out Robin but by this point, Robin has realized she's much more interested in Nancy and politely turns her down. Robin then goes to Nancy in some sort of "the world's ending so I might as well say it" thing where she fully expects to get rejected but instead they kiss instead and the series ends with them both together.
In terms of likelihood for each scenario, I think that Ronance is unlikely because of the fact they're really pushing Robin and Vickie as well as the whole love triangle thing with Nancy (I hope to be proven wrong though). But in terms of the other options with Nancy? I'm not too sure. Single Nancy doesn't seem likely because they're pushing her into choosing between Johnathan and Steve. Jancy is so cute but they just can't communicate and I don't get what the point of Steve being all flirty with Nancy in S4 would have if they weren't pushing for Stancy. I also think that they're planning on killing off Steve so giving him a love interest would make it even sadder.
Thanks for the requests! Hope this was interesting. I can make a follow up one with Robin and Vickie if anyone's interested as well.