This is a common thought of mine.
I play a game every so often called Mount&Blade:Warband. There is a mod called "Persistent World" in which you are thrown into an MMO sandbox-type world and you're essentially left with the standard fare choices to make. You can collect, craft, be a doctor, knight, whatever. Upon death you lose everything (though you do respawn) and are asked to roleplay being a new character. While I am not a fan of roleplaying, I agree to keep up the atmosphere.
The mod is fun, you have bandits, ransoms, duels, wars - it's all very "Persistent". But it had me thinking. This game would be reasonably more fun with absolutely no latency and a more surefire way to ensure that all players keep to some basic rules. At this point I was back to thinking about AI again, if it could match what an online player contributes and still has the same unpredictability.
What games have excellent AI? Do you think singleplayer can give the same multiplayer experience? If anyone plays the popular competitive multiplayer games (TF2, Halo, etc..) I'd love to hear your thoughts also.