I have looked at a range of cards in both card shops and online to see what type of design is popular at the moment, most of them being simple and either illustrative or collage cut out style designs. Most cards seem to look better when paying attention to detail, or when extra touches have been added such as foiling or 3D paper elements.
Another thing that I found quite popular within card designs were the use of brown stock, especially for envelopes. I think this is something that works well, especially against black text.
For a range of my cards I would quite like to go into card design within music, popular cards are sold with recognisable album covers on that take up the entirety of the card, like The Beatles collection below. I think it would be good to incorporate a musical aspect, either lyrics etc. into a set of the cards.
THE BEATLES - BIRTHDAY SONG