The cards using the same art assets from the TF2 map title screens. On the TF2 blog they're talking about map updates and new maps being made official. So based on that I'd say the cards will be a set based on TF2 maps.
Or, they'll be sets of cards based on maps/locations for multiple games...
It seems most people agree that the lose of the beta is no big deal. I, however, am curious about what this means toward the future of tf2. You can see on the Steamdb page the beta was still being updated as recently as a month ago. Also, it appears there is a closed beta listed under the betas...
Yesterday Valve removed the Team Fortress 2 Beta from Steam. Below is a link to the Steam Database entry and thanks to wickedplayer494 on the tf2 subreddit for bringing this to everyone's attention.
http://steamdb.info/app/520/#section_history