I don't know if the studio that makes GTA has ever had power autists of RDR extent working there long term but if they did we'd have that and more. GTA IV I think felt more alive than V, it felt like the world was actually moving and you were just a piece in it, which was especially impressive given the hardware limitations of the time. You could be driving along the bridge and spot cops searching cars, or have actual police encounters happening, etc. The NPC cops had better AI and instead of just gunning at randoms they used cover and avoided shooting if possible. Street vendors, pedestrians, etc all seemed to just be going about their days in a way that V tried to mimic and it just came off as fake and as though they're all performing for you.
Imagine a world like that set in Vice City with the attention to detail that the RDR spergs had and the story writing of the classic games, it would be the actual best game of the decade and not just "well at least it's not another remake"