I've tried playing two-player Settlers, where each player controls one set of pieces, and each time a player builds they need to play a piece of the same type for one of the two uncontrolled colors (so there are two non-player nations in play).
What I would like to add to this is for the uncontrolled nations to have resource cards and some mechanic for them to allow trading with the players in the game.
Are there any two-player rules that support this?