I definitely agree this would be a good idea. It obviously won't be too similar to RDR though because of the different settings. I definitely love the idea of bag snatches; you can choose to return the bag or whether to keep it.
You know, they could probably add drug deals and stuff like that to the mix. The player spots a drug deal happening in an alleyway and can choose to ignore it or to kill everyone and take the stash and money. Brilliant. This could then develop into a deeper minigame, say the dealing from Chinatown Wars for example, but that's another story for another day.
I'll try to think of more but I'll leave it at the one for now.