A new idea that will hopefully be found enjoyable by many on the site.
They can be used both for trading and for Summoning in the Battle Arena. Take this example here:
Obviously, we have the avatar at the left, and then name, rank and village. Then is the special technique that can be used in the Battle Arena (once per battle). Under the avatar is the Attack and Defense power of the Summon. When this is used instead of the Special Technique, the attack is added to the attack power the user already possesses. So if a Genin uses it, the normal -10 damage will be added to the -19 damage to do -29 damage. The same works with the defense, meaning the Summon can take away -17 damage from an oncoming attack. The barcode at the top indicates the rarity of the card. The lower the number, the rarer the card probably is.