If you've played The Elder Scrolls IV or V, you know how awesome they are. You'll just be strolling from point A to point B, minding your own business, when suddenly you're jumped by a bandit or thrust into the middle of a skirmish. Even Red Dead Redemption had a few of these random occurrences. Stuff like this would work perfectly with a game like GTA. Putting them in multiplayer as well could also be awesome, especially if there's rewards to be reaped from participating in them.