My idea is an abstract tile placement game for two players. Each of the 18 cards has on it a unique pattern and also a unique scoring tariff. The patterns are based on three variables [red|blue|green] [circle|square|triangle] [hollow|solid]. I will probably put some theme around it if I get as far as making it, like cuddly animals or whatever, but it is basically abstract.
Shuffle the cards. Put one aside without looking at it. Deal each player one card, face down. This card is that player’s scoring tariff for this round of the game. Put the other 15 cards in a deck, face down, to one side of the playing area.
Play alternates. On their turn, a player may take the top card from the deck and play it onto the playing area, or may move a card that’s already on the playing area into a different position. But they may not choose to move a card if they moved a card on their last turn: instead, they must draw and play one.
Cards must be played/moved adjacent to at least one card alrady in play, and in a notional 5 * 3 grid. The edges of the grid are undefined, so early in the game you can play cards in any direction, but you can never play (or move) one such that a grid to contain all the cards in play would have to be larger in either dimension than 5 * 3.
[Is it clear what I mean there? This is really difficult to explain without diagrams…]
The game ends after the last card has been drawn and played, at which point there will be an actual 5 * 3 grid of cards in place.
Players then reveal their personal scoring tariffs, and count up their scores.
I haven’t yet worked out exactly what the scoring tariffs will be, but they’ll be things like: “score 5 for any 3 reds in a row, score 3 for any 3 hollows in a row, score 1 for any pairs of circles next to each other”. When a particular scoring tariff is in play, the pattern that’s on it will be out of play, of course. And there will always be one unknown pattern (and scoring tariff) out of play, so you can never be quite sure exactly what your opponent is trying to do.
Repeat the whole process (say) 3 times and add up total scores to determine the winner.
This might turn out to be quite a dull game (as they so often do…) but at least it has the merit of being quick, cheap and easy to produce prototypes.