I feel that you still require some form of visual to do a podcast for cards.
At the end of the day, collecting playing cards is slightly different from card games or board games and the like. For the latter, you can talk about the gameplay, the accesories and the rules, maybe even some strategies and such, and it is easier to be formed up in someone's mind.
It is harder to talk about the design of a card and expect it to form up in someone's mind the way it is meant to be. You can talk about a intricately designed dragon back, and while some may visualize it, others may visualize a cartoon dragon from Disney or something their 3 year old drew. You can say the deck fans better or shuffles better, but how much better or what is the difference? I say it is harder to transmit this information over audio.
As far as topics - experiment. Try different things. You might discover something your audience never realized they'd like, but they like it a lot. You'll never learn that by taking a poll. Design by committee is usually a recipe for disaster, so I don't recommend using it for determining your podcast's content.
I totally agree with this. Talk about everything card related. History, present, fun facts, design, handling, manufacturer, manufacturing process, pricing, current market trend, card games, card accessories, books, artist, designer, finishes, auctions, magicians, magic tricks....the list is endless. Slowly filter out what your audiences like and do not like, and from there you could have a very entertaining podcast.