There are multiple possibilities for a God of War card game. It could turn into a simple adventure or several Kratoses battling it out across the earth. It could easily be made into different eras that one or several games take place in, and, of course, you can customize your own deck/character to fit that era.
The cards that you gain can be power-ups, along with monsters that you fell during your adventures. Along with the many unique stories that are still to open with the Norse God of War, there are many possible storylines they can follow and many cards they could create, as many as there are myths and legends.