I've always thought of Catan as the game of colonialism, and Risk as what the imperialist powers do once everywhere has been colonised.

Diplomacy is the game of what happens when the Great Powers go for each other's homelands directly rather than just their colonies (great game, but you WILL lose friends over it !)

I've always thought it might be interesting to update risk from the 1900's to the 2000's, with the addition of thermonuclear weapons. First person to use them automatically loses, and the "winner" is the last player with any population left.....