Sometimes I come up with card ideas that I'd like to test out to see if it was a good idea or terrible idea, but don't have the cards. For example, yesterday I spent 10 PC on a 1 star to test something out and it turned out to be one of the stupider ideas and failed really hard. So I wasted 10 PC for no reason. I really wish I could get my 10 PC back.
My suggestion is if their could be a Test Arena where you pay FM (or nothing) to "rent" a card for about an hour or so and fight NPC or other people in Test Arena.