So this is completely up to you guys as developers to implement but just plain and simple, I feel a coop option to game would be amazing, it would be great to play this with friends and I feel you already have a great feature in place for this.
By that I mean your posse system, simply, a player who joins another players session could simple take control of one of the recruited posse members, the next part is up to you and how you feel it would best be balanced. Either you could give these posse members the same abilities of the main character or just simply let them have guns/armour (maybe talismans). Personally I am happy with either of those options.
I am not sure if it is possible to incorporate something like that into the game now its released, I dont have that kind of knowledge but even if it is a smidge possible I would recommend to at least have a look down that path, if it isnt no problems, the game is still great anyways!
Also if it even has to be a standalone thats fine!