The only swaps I've been in are those where you select a focus fabric, send a 12-14" square of that fabric to the other quilters in the swap (usually 12 to 16), and they each make a quilt block for you, incorporating your focus fabric. That way you do have blocks that have at least that one fabric in common, and here's an example of what I was able to do with the blocks received in one of those swaps.