Rockstars cowboy epic with new pc enhanced graphics out early next month
It'll include their online mode Red Dead Online
Doesn't look like it will be coming to steam at this point instead epic or Rockstar store
It looks like if you go with the rockstar store you get a free upgrade to either the special edition or the ultimate (not sure why you wouldn't just pick ultimate as that should include all the stuff in special edition)
 I looked at it and it works such that if you buy standard you get special or you buy special you get ultimate [/edit]
Special edition gets you another mission and some cosmetic stuff, ultimate all of that plus more cosmetic stuff
Not sure what the online is like if it has SNG potential or not. The single player is pretty great though just in it's own right.