I have entered the BGG 18 Card Microgame Contest!
The idea of making a complete game with only 18 cards in intriguing and challenging. I've developed a fairly abstract and simple game called CMYK, which you can download and try out.
Basically, the idea is to make lines of cards in your colour, horizontally, vertically or diagonally. The twist is that whenever you play a card into the grid, it swaps with a card already in play. This can lead to some very tough choices and sticky situations when the grid fills up. Because this also means that if you go first or last you have no choice in what you do, the game is played over four rounds, with each player going first in turn and the winner being whoever scores most points over the whole game.
I'm also playing with the idea of doing a second game, somehow involving combat, but I have not gotten any further with it yet. I think it will involve rotating the cards to track damage and hit points, but beyond that I know not...