If it takes place in the wild west I'd like to see basic, slow cars emerge in the world. Maybe a gang vehicle or caravan? They're slow and breakdown often and you have to fill it with gas as a downside. A positive side is bringing gang members with you in the car and its faster than horse and carriage. Or maybe put an early repeating gun mounted on the back. I'd like to see trains take a role in it with as much importance as they did in syndicate as well. Rails did build most of the west.

In terms of weapons, more evolved revolvers, carbines, shotguns, maybe explosives come back with dynamite? They could totally return the wanted system with posters and paying off ransom with old wanted posters and cops.

They could make this the most looney tunes crazy wild west shit ever and I'd still love it. It would be like what I thought when I saw black flag, I had no idea how they'd pull off fucking pirate assassins but it happened gloriously and it was fun an engaging. So hopefully cowboy assassins happen sometime soon.