In the land of Ouzina monsters roam the land freely, forcing humanity into walled cities in order to protect themselves. For years it seemed that there was no way to fight back the creatures, until one day a Magi created a new weapon against the monsters. He named the weapon Caster Cards, and when they were used the cards could capture the very souls of monsters and covert them into weapons for the use of humans. With the Caster Cards humanity began to fight back against the monsters, gaining ground and finally expanding past their walls.
As time passed groups of Caster Card users came together to form Guilds, which under the direction of the Unified World Kingdom would go on quests to hunt down monsters and aid the people of the land. The story begins with a new guild gaining their certification and building a name of itself.
So thats the setting, here is how the game would work. Guilds would be controlled by the players together, sort of like voting. When the game first starts out each guild member will need to go on missions, but once NPC's start joining players can also have their characters sit back and take care of more mundane tasks, like directing the NPC's or building up new cards by fusing them.
Cards work under this principle. If a monster is defeated and a player has a Caster Card on them they can absorb the monster into it. There are three kinds of cards, Weapon Cards, Augment Cards, and Summon Cards. Weapon cards turn the monsters soul into weapons players can use. Augment cards give the player special abilities they can use. Summon cards allow players to summon weaker versions of the monsters to fight for them. However players need to be careful, because the stronger the card the more difficult it is to control.
Missions will be given to the guild that they can choose to go on or just ignore. The more missions the guild takes the more their standing with various factions will grow, and they will be able to take bigger and more important quests, which also equals getting new and better Caster Cards.
So thats the basic idea of it.