Ok, here's a few suggestions..
Pets! Why not make Dogs, Cats, Bears, Cougars, Eagles, Coyotes, all tameable as pets?! How awesome would that be. Could use them for different things, like hunting, protection, companionship, etc. I like the idea of someone ganking me and then my Grizzly chasing them and ending them because of it, haha
Citizen's Arrests! Make it so you can hog tie someone and take them to the Sheriff, if they have a Bounty on them. Need to make it more difficult for people to unhogtie themselves, obviously.
Brothels! It's the Wild West, I mean, come onnnnnnnn. I'd love to know why Rockstar didn't put this in. Crazy move. It's the one thing I was certain we would get.
Exploration! More bonuses and rewards for exploring and hunting and stuff. Make lots more secret areas and weapons to loot off ground of NPCs you can find, oh and add more weapons. Bring back Trapper, so hunting a certain animal gives you cool stuff. Bring back legendary animals, but put spawn as random, and only one active per server per time, etc.
I have loads of suggestions, but this'll do for now