Obviously, I love your cards as I continue to purchase them. My suggestions: consider a few cards age-specific - e.g. 50th, 75th, 80th.
Take a look at the Paper Love website, particularly the bike with flowers and the Christmas card with the bird dressed for winter. Notice the intricacies of the bike and bird specifically! I love your Yellow Submarine card - colorful and lots going on and overall, prefer your cards to theirs, so consider these as little tips to outshine the competition!!