The 18 card limit really drove both my motivation and the design. I spent a few days thinking about ways to overcome the limitation. As I saw it, the problem was that you need a relatively complex game state and provide a sufficient amount of options for the players for the game to be strategic, and also to have hidden information to feel like a card game. And you only have 18 cards to represent that. I settled on using the cards placement on the table in a grid pattern as the game state. So the players place cards on the table to build up the state similar to games like e.g. carcassonne. Also, the final score should be given from that state alone so as not to require any extra tokens.
For card placement to be an interesting mechanic cards should affect nearby cards, and with only 18 cards all cards need to have multiple functions to give enough possible interactions and combinations. I chose to go for a model with cards granting one of three different resources (gold, food or knowledge) depending a condition, and also being of one of three kinds of cards (people, buildings, land). This makes the cards not only score on their own but also affect the scoring of nearby cards.
Now to give players more strategic choice and to have hidden information and also to feel more like a card game, players get to put cards in their hand. Again, 18 cards is not a lot so you cannot have too many. I chose to go with a combination of a 3 card hand and 3 cards in a common pool. This gives a total of 6 cards to choose from, and still have an initial remaining deck of 9 cards. It also has the added complexity that you see three of the cards your opponent gets to choose from next and may pick one defensively.
I think the game turned out to be a lot of fun and quite complex to play. The interactions worked out the way I wanted. My biggest concern is that the scoring gets a little complicated. I'd need more playtesting to see if it actually is strategic or not.
I haven't decided if I want to pursue it further yet. We'll see. Given the limited time I pretty much completely ignored the graphical side so there is quite a lot of work left. But I note that the low production cost for a game with only 18 cards opens up some interesting possibilities, such as bundling and promotional giveaways.