This is a very nice idea that puts large amounts of replayability into the game.
But I think that this could be taken a step further.
Here are a few recommendations for future updates. It's up to you if you want to implement them or not.
I think that the next thing you need to do with this is randomize the cars that the AI uses, and make the AI cars use paramiters (.con files) for the car that is randomly chosen, not the parameters made specifically for that mission. (for example, if you are doing a race mission, such as S-M-R-T or Getting Down With the Clown, and the F1 car is chosen, you are going to have a tough time, but if the donut truck is chosen, it will be really easy, and for a destroy mission, if the speed rocket is chosen, it will be very easy, but if something like the armored car is chosen it will be very hard) This would add lots more replayability to the game, and makes the difficulty unpredictable as well, like how randomizer tools for other games such as Pokemon and Mario do.
Also I think that it would also be a nice touch if you could randomize the traffic cars and the bonus cars as well.
Again, these are just some recommendations. It is up to you to decide whether to implement them or not.
Also, I'm not sure whether this should be considered a bug or not, but when I was doing the mission "Petty theft Homer", the car randomly chosen was the Family Sedan, and there were two Homers in the car! One in the driver's seat and another in the passenger's seat.