Sell it or give it to a friend. I'm happy enough with my town, and I don't think I could maintain two at the same time. Also, while I appreciate those who use a second copy for villager giveaways, I don't see myself having the patience to do the same thing, even if I were to just look for villagers that I want.
I'd buy my little sister a 3ds and give it to her because she's always asking me if she can move my person around and it's so damn cute...but at the same time I'm like no, go away.
I...might use it to get the zodiac furniture. And such. Its also another turnip price town which reduces the risk of losing money. Also could use it to find villagers I want in my primary town. All that "dark side" stuff.