I've got an idea for an open-world Stealth FPS, but I'm not sure if the AI technology of this generation would be able to support it convincingly.
You're an average Joe out on a shopping trip with your girlfriend (let's just call her Cindy). The first thing you do is go with her to a laser tag place near the mall, and the two of you team up against some teenagers for a couple of rounds. This is both a setup for the relationship between you and a tutorial for the shooter mechanics.
So anyway, after the laser tag, you go with Cindy to a store in the middle of the mall for some shopping. It's the middle of a saturday and the mall is really crowded. Then suddenly BAM! Explosions and gunfire, people running around, confusion!
There's a large force of about a hundred terrorists assaulting the mall. Some of them are roaming through the mall shooting anyone they come across, and some of them are blocking the exits off. From this point on the game becomes entirely open. No mission objectives or anything, you just need to survive. You can just find a way out (Cindy follows you, but if she gets killed she's gone forever) or try to rescue as many people as you can, or go all John McClane on the terrorists.
There would be events that would happen on a timer, and the whole game would be only about two hours long, at the end of which a SWAT team charges into the mall and sweeps up the remaining terrorists. If you found a way to escape early, then you would effectively save your own ass and end the game. Or you could find the escape, and tell Cindy to run for it while you go and find other survivors or shoot the bad guys. Or you could give her a gun and she'll help you out. The AI that she has would be similar to the AI of the survivors in the mall, except some would be more or less willing to help you. Some just want to escape, some want to help fight, some are intentionally programmed to freak out and screw things up for you.
The enemies, though limited in number, would all be very smart, and they'd all have cell phones so they can communicate with each other so you'd have to extremely careful when you try to take any of them on. Since it's just a mall, ammo would be limited to whatever the terrorists and security guards were carrying on them. You would also benefit greatly from taking out the terrorist that's posted in the security room watching the security cameras. Or just shoot out security cameras whenever you see them, or find the circuit breaker to disable power to the security room and shoot any terrorist who tries to restore the power.
If you manage to get rid of terrorists who were guarding a certain exit, you could then tell survivors to run out that exit.
The game would have two hours of video and radio news coverage that you could listen to any time you pass by a TV or radio (like, in an electronics store or something).
About 30 minutes into the game the terrorists would switch from killing people to taking hostages and making demands on the news.
If you killed a significant number of terrorists, then the remaining terrorists would know about you specifically. The leader would go on the mall intercom and threaten you. A few more terrorists killed and he'd start giving ultimatums like give yourself in or he'll execute a hostage every minute. If at any point Cindy got taken by the terrorists, they'd use her as bait in an extremely difficult ambush situation.
Instead of taking out the terrorists directly, you could use your cell phone to give the police intel on the situation so their SWAT attack would go more smoothly and less hostages would get killed in the crossfire than otherwise.
It would probably take several runs through the game to get a "perfect" ending (rescue Cindy, rescue as many survivors as possible, and kill all the terrorists)
I'm sure there's all sorts of ways this idea could be expanded on.