Daniel Solis has written a quick but extremely insightful and well-illustrated post about the relationship between package design and card design on his blog.
It’s all about Iterative Visuals. Start with one baseline, change one or two things, and always only change those one or two things for each iteration.
The game design and publishing community can often become a nonsense-spewing echo chamber, but Daniel’s graphic expertise from outside the business often makes his insights especially valuable to us in the game industry, in the same way that I think lots of domain knowledge from software development and product design can be.