A stage show with cards? i know im not helping but really, stage show with cards? i have seen only two people pull this off, Tamariz And Ricki Jay
I'd say you need to study your magic history a bit. . . Harry Houdini was known as the "King of Cards" long before he was known for his escape work; the Great Cardini was one of the more amazing acts in magic, much of what he created is still being used by traditionalist. Thurston and several others were known for scaling cards as is Princess Tenko in today's world and I believe Jeff McBride.
There's more than 100 card effects on the market that use over-sized cards so they can be seen by a theater styled audience and I couldn't begin to list the number of card routines that were deliberately created for the stage, including some rather expensive gadgets, balloon holders, cute little duckies that remove the chose card and more!
It is very possible to do 20 minutes with nothing other than playing cards and yes, you could be entertaining. . . not likely, but some people can pull it off (far more than you'd think . . . ever watch Max Maven?)
I loathe playing cards and 99% of all card effects because
a.) too many illdisciplined magic lovers refuse to learn how to present them properly, let alone in a manner that is clean and polished;
b.) far too many of us are addicted to the things and rarely look beyond the length of our own deck, to find an alternative piece of material that would more likely prove even more bewildering;
c) magician's lean on playing cards because they are portable and easy to carry around, which is the wrong reason to consume yourself with them . . . by learning proper slights and force your mind to be creative using everyday items you will evolve in the public eye as being a "real" magician vs. the guy that has some cool card tricks. While many see me as an A-hole for holding to this POV many more have thanked me for waking them up and seeing beyond the deck.
I bring up this "issue" of mine for the sake of perspective however; if I can see how to do a 20 minute stage act that is nothing but playing cards given my loathing of the things, how much more do you think a lover of card magic can generate?
A SHOWMAN can deliver a show with nothing. Magic legend John Calvert once did a two hour show with a borrowed deck of cards, some rope and if I remember the story right, he had a chalk board. It's the kind of thing that makes you a legend in the minds of the public vs. your own delusion.
Enough. . .