Based on how GTA5 vs GTAO evolved, my prediction is that they will take what they have learned from that and adapt the experience for RDR2. What I am talking about is this:- When GTA5 was first launched, there was the promise of DLC. What actually happened was that this plan was scrapped and all development on the SP mode ceased. Online had map changes (additions of bunkers etc), new guns, cars, clothes etc etc. This tells me that (had they known this from the start) R$ would have developed the relationship between online and offline differently.
When I say that I there might not be an obvious divide between the two, I mean like GTA5. The SP mode has been left behind and is now totally separate from online, which I'm certain wasn't their first intent. If you remember, they were releasing cars etc into SP mode as well as online, all the way up to the introduction of Benny's, so that tells you that it wasn't part of their original plan to separate it.
Like I said, I think they will learn from that and online vs offline will be different to what we have in GTA. Not sure how they would do that, maybe you're already in an online lobby when you're playing SP, maybe there will be a single map for both online and offline and seamless interchanges between the two modes. Maybe in SP you can start an online mission. Not sure, but I'm betting it won't be the same as GTA.