I think it could work, if it was marketed properly. Throw in something like rare foil cards with scratch-off codes that net you a significant sum of credits, and many existing UR players would purchase them for a chance to win. Keep in mind that they could be sold off the site as well, and not just in stores... So they would already be exposed to a large customer base.
Me and my brother make cards based on anything and we print out coloring pages and shrink them to tiny size then color them lol. We put them into a protective case and make ability and bonuses. We actually made some ability's that UR uses now. Like heal but we called something else. and copy opp bonus is another thing we had so yeah. But we also found that gameplay doesn't really work because so much of the fighting is not knowing how your opponent plays or what cards they are going to have.