I was thinking it oculd be another option for a combat system- similar to a boardgame and digital game on Steam Talisman, where you have Atk and Defense stats when you go up against an opponent you roll a die as a modifier to an attack- if the character has a piece of armor or a weapon this gets tallied into the equation as well and thats it. 

For other ideas for uses of cards- weapon and armor sets could be on a card with their stats, and cards could act as keys as well, like one card could be ("A drunken off-duty night guard once gave away a password to a good friend of mine."- Use this card at the lower gate of the Black Castle) or something like that. Another might be an invitation to talk to a blackmarket dealer the card vouches for the PC etc

Cards could denote skills- Your perception for traps has increased double. (Which means booby traps will be visible in the game- where as some might be considered hidden with just a trigger in the game that the player might set off other wise) and with this idea the player maybe can only carry or have active 5 cards- so some might boost stats or be a foot into the door in some place another might allow the PC to regenerate full health once etc. 

Card Artwork with RPGinABox:

The way we create an image for items for the inventory could be used on the card applying a basic image along with a background design either in templates form which we add onto or from scratch. Perhaps a editor for a card face could be a vector shapes editor with layers and we can give borders (thickness and style templates) and fill and a text box with perhaps two small boxes on each bottom corner for stats. They all could be universally the same template but the player can alter the design and import an image the same way for inventory items. This way games will look different from others in terms of card designs. They wont be flash as large budget card games- but would fit the game i think if you have basic colours and layers to stack shapes in a way to create a pattern or image (the racing game Forza is a great proof of concept for how basic shapes along with layers can create amazing art- but i am not suggesting having something as complex as that- but along those lines). Alternatively the user might have an option to load an image they created elsewhere and place it on a blank template and then move two stat boxes to where they please that is the top layer. This way those who are good at art can create something with their own tools if they want to. 

Here is some examples ive made as concept cards for a text adventure game i was making but never finished..i used Paint.net to create it. 

The borders and text boxes could be something the RPGinABOX could have some templates for to create shapes and layer them within the game editor or the user could important the image into a paint progam and make something like this. My card concepts below use a photo image in the background. These examples below could allow to have some gameplay events in a simple way- such as gun jam or being ambushed out of nowhere and not see the creature in the game at all- it could just be an event that twittles down your health or morale etc.. 

Some battles will involve enemies present and animating a menacing pose or something and we use the battle scene or room, and the cards doing the battling in the foreground- perhaps there is a slash, sparks, explosion effects built into the game that can be assign to card types when a battle concludes after two cards are revealed and a die roll to modify stats where you might get +3 attack and the enemy only gets +1 giving you the higher attack of both cards have the same attack strength.  

Also if the player has a small party- cards could reflect combat strength so it might be combat strength vs combat strength of opposing side.