Rockstar/T2 would serve as a good enemy.
With all honesty, though, forced co-op wouldn't do any good to public sessions, already being the very gutters of this game, filled with feces, cheaters, blood, money begging kids, used condoms and tryhards. You can't expect these players to cooperate with normal people, who have their own goals - I/E missions, biker club supplies or just want to hang around with others. I mean, if they don't know how to get a solo public session, that is.
The problem is, public sessions are meant to be free - you do whatever you want in them. Because of that, having a major goal, like a common enemy, would attract both players wanting to participate, and people who want to ruin their fun. You can't have freeroam AND proper mission objectives at the same time.
Now, do I support the idea of having more co-op activities? Absolutely! Heists was the first and only update that not only encouraged people to play together, but has left a long lasting impression on the community. As profitable as I/E is, for example, its public session nature achieved the exact opposite, giving people a valid reason to play solo, where's no cheaters and griefers.
Lowriders update introduced few missions, all of them fun for the most part, but poorly executed, allowing unprepared players to start them, and then die over and over again. Yet, once you pass the first two or three, they can be really, really enjoyable.
The key is, neither of them takes place in public sessions, where someone from the outside can interfere and ruin the fun for those who try to complete them. Session consists only of participants, having the same goal.
That's something I'm looking for in this game - objectives, goals, reasons to actually play it and cooperate with other like-minded players. Once I've got more money than I'd ever need, completed Criminal Mastermind and all contact missions, GTA Online became empty and boring to me. I've just burned out after doing them countless times over and over again throughout last months. Yet, R* has put emphasis on PvP and money gambling in freeroam (special cargo, IE, bikers, gunrunning, smuggling), changing the direction of the game, leaving it completely different from what it used to be, and what this franchise is (was?) all about.
Give me co-op activities, pretty please, but nothing involving this entire cursed freeroam I'm sick and tired of. Proper missions, preferably telling a mini storyline, like heists or Lamar's lowridering, leading somewhere, to a bigger outcome. More worthless, ultra repetitive tasks, like supply runs for bunkers and biker clubs, or cargo sourcing for warehouses and hangars won't revive my interest and love for this game, and so won't more freemode events.