For 2018, how bout a Color My World theme. I will pair two colors a month, drawn randomly, and u will make a postcard using those two colors. you can add another color, but the main theme must center around the colors of the month.
Other ideas were
Wacky Holidays
Picking two letters a month and designing a postcard depicting one of them o
Artsy theme such as wavy, circular, straight, landscape, stripes, dots, stars, and then i am stuck for other terms.
Let me know your thoughts.