I use a number of fan-made variant card decks to play various games, on Roll20 and in person. For in-person play I need to order custom decks of cards from an online print company. But for play in Roll20, I only need the images, and I can make any deck I need, which will be easy to use due to the robust card deck features now included in the Roll20 interface. The upcoming "Update of Holding", will include a new Roll20 iOS & Android app, which got me thinking. Why not produce a smaller companion app, just for simulating card decks on your smart phone? As it is now, there are a number of flashcard apps available, which could do the job with a lot of work. However, I have not found one that will let you upload images of cards and backs and create simple custom decks like the ones in the main Roll20 interface. If such an app could create and keep track of multiple decks, it would be a great resource for many tabletop games.
