So it seems to me that Rockstar have left themselves an easy gold mine here with the way they've built RDR2. Aside from the Mexico region (for which the assets may even be mostly done), the entire map of the first game is already here, a bit less lively - granted, but nevertheless mostly all there. It wouldn't take all that much work, I imagine, to simply add Mexico, throw in a time jump that spruces up New Austin just a touch more, redo the old cutscenes from the first game....and you pretty much got yourself a remaster hooked right up to your sequel.
This would be quite a clever thing to do really, because you make a lot of people happy that are hoping for single player DLC, but at the same time, you can devote the majority of your resources/attention to building up Red Dead Online. Given the fact that Online was pretty much their only focus point with GTAV post launch, it wouldn't exactly be surprising if they wanted to do the same here. It's a big money maker after all. Remastering the first game within the groundwork of what they've already built here would be a relatively easy way to appease the single player crowd, at minimal cost.