Something similar to FF 8 or 9.
You could make the starter set cost 50k-100k (or even more) with the possibility of booster packs.
There could be a leaderboards and tourneys. Which would net you better cards and/or unique gear. But the best would be just running up to someone and challenging them. If you win, you would get card points to spend on gear/rare cards (and knowing SE, these items would cost a lot of CP.
The last little bit would be that every enemy would have a chance to drop their own card. Of course the drop rate would be low. And even lower for rare baddies.
Thoughts? I know not super thought out, but the idea popped into my head, so I thought I would share.