It would be kinda hard to make it a co-op game. Sometimes you may not find a person to play the whole game through, which would ruin the parts in where you are separated.
But if you are talking about making it just levels, or rounds, then you would have to make each round or level unique and it would have to be different every time, so you don't get bored. So I suggest the rounds being randomly generated to create a different experience.