I'd love to see a side story showing the battle between konoha and kyubi, how it started and details on the actual fight.
Second choice would be tha fall of orochimaru, obviously the 3rd trusted and respected him at some point- i'd loce to see a story about his fall from grace, what made him turn evil- not just a generic "i want it all" supervillan moment, but a real look at what motivated him to become so powerful and evil- I really get the feeling there is somethign more there.
Failing that, a gaiden about a naked drinking competition amongst all the female ninja's when all the men are out on missions.