Here are few ideas to add to yours : (card vendor npc) - all actions below are interaction with npc.

- card can be sold at card vendor npc for gil
- combination of cards (or more of same card) can be exchanged for crafting material (no gear! but material)
- some type of cards can be transfer into shards or crystal
- card vendor npc sells basic lvl 1-10 cards for gil

Play with npc
- you could be able to play with some npcs in around the world or in the cities
- some of them would have expensive cards, but hard to get. Skilled players.

Game win
- you can take up one card from the enemy

Game lose
- enemy takes your card

Another
- there is tournament held in any of big cities, where special rules will be implented
(example with lose you lose more than one card, etc.) players can interact here.

Interaction
- you can play with any other player, you can lose or get card

methods to get a card
- monster drop
- nm drop
- play with npc
- buy basic cards at npc card vendor
- via gathering (but this sounds weird and I dont like it personally - only if % was really low)
- leve reward
- quest reward