Coran Stone (a.k.a. Kizer180) envisions a grown-up Calvin and Hobbes facing off against a grown-up Christopher Robin and Winnie the Pooh in an art series pointed out by io9. Stone was even kind enough to explain the backstory behind the illustrations:

How Dr. Robin and Secret Agent Calv know each other:

As children, these two had constant feuds after Calvin literally stepped into Christopher's neck of the woods. That's when the Battle began, the test of their imagination. Calvin would talk about his "friend" and his abilities and Christopher would counter and say "but can he..." it went back and forth between the 2 of them. As they grew older they grew in different directions. Calvin met new people and made friends – although his best and imaginary friend "Hobbes" stayed with him all the way into adulthood. Christopher on the other hand became more introverted but made the characters he invented as a kid more apart of his character as he got older.

Why he's Calvin's Nemesis:

Christopher Robin had to leave for boarding school after being accused of burning down the 100 Acre Woods. Later ol Chrissy boy learned that someone saw a stripped shirt, wearing kid also wandering the woods on that very day. Turns out that Calvin who was fond of playing with matches at the time burned down most of the woods by mistake – Chris got the heat though.

That sucks Balls!

Read more on Stone's DeviantArt Account.