Yeah, I totally get where you're coming from. RDR2 is already one of the most immersive open-world games ever, but adding romance options—especially ones that let you build relationships with NPCs beyond scripted interactions—would take it to the next level. Rockstar put a lot of detail into the world and its characters, so it's surprising that Arthur (or even John in the epilogue) doesn’t have more dynamic romantic interactions beyond Mary Linton’s storyline.
As for whether something like this could happen in the future, there are a few possibilities:
Official Content from Rockstar – Highly unlikely. Rockstar doesn’t typically revisit single-player DLCs (sadly), and their focus is mostly on GTA Online and whatever GTA VI has in store.
Mods – This is the most realistic route. There have been some impressive mods for RDR2 (like new missions, character overhauls, and even Undead Nightmare-style zombie mods). A romance mod would require a lot of work—new animations, dialogue, AI scripting—but it’s definitely possible. The biggest hurdle is that RDR2 is harder to mod than games like Skyrim or Fallout.
Community Crowdfunding – If a talented modding team were to take this on and got enough support (financial and otherwise), something could definitely happen. If you’d be willing to throw money at a project like this, chances are others would too.