I like the idea of being able to buy a customized card (not a randomly generated card). To make it available to non-paying players one could use the fey tower and aetherstone (with a guaranteed success). The way it could work is for every line you choose, enhancement, and upgrade you pay aetherstone. For a simple card with five lines selected the cost would be 1 mil aether (city max) and any additionally required aether would have to be purchased with gems. While this would still greatly benifit a paying player as they would be able to purchase a custom wondrous +10 card with enough gems (they could also cap the upgrades available this way and the players that would spend that much are already spending that to get the master tokens for the upgrades anyway) I find this much more appealing then having to sort through thousands of cards to find the stats I want and then enhance and upgrade the card from simple anyway.