Killing everyone on the map with contagion sounds like it would be fun for 5 minutes or so and just imagine hitting someone with suicide. Better yet imagine you just spawned in and all of a sudden you're pulling out your pistol and putting one in your noggin and no amount of key mashing will stop it. Or imagine you can't see, or your weapons will not work, etc. It's not just the quickhacks though, you also have weapons that shoot through cover or can go around. There are a literally thousands of sniper perches. Nerf everything and it's not Cyberpunk anymore; it's some guys who could be in a Quake 2 deathmatch except it's taking place in Night City instead. Co-op does sound more likely, it would allow you to keep the arsenal and the balancing would happen more on the machine side. You could create raids, that sort of thing. BrightSmith may be on to something there, that could work.
