The Ossiranth Chronicles

Inscription has the ability to make the Darkmoon cards, which can then be made into decks and traded for trinkets.

Why can't the whole idea be expanded upon to be more than just a gear treadmill?

It's been done in single player RPGs before, FFVIII coming to mind most readily. Collectible card games. It would add yet another PvP aspect, or even PvE aspect to the game. Collectors can earn prizes for completing sets and collections, there can be tournaments with prizes for winning, and it could even play into questing with needing to challenge some people to card games to complete objectives or advance a personal storyline.

This also allows more to do for crafters.

There's also the aspect of external card games and potential interchangeability between them, but that's an entirely different post unrelated to in game mechanics.

I think the idea has potential if it's done right, and so far I can't recall any MMOs that have done it at all, let alone correctly.