If a developer keeps it in mind when they are designing it then it will be designed for it.
Sorry, but positive thinking doesn't cause co-op support. This is something that has to be considered at engine level from the start, unless you're happy with something like Far Cry coop, Oblivion multiplayer, the various HL2 mods, LMS for Doom3. Which are all well and fine for user-created mods, but absolutely not at a retail level.
Mind you, most of the mods up there were developed across a number of years, with easily hundreds if not thousands of man-hours to reach that incomplete level.
The PC has enough shovelware-style multiplayer. We don't need anymore.