-When actually in-game, set up a span with a z-index below the canvas field (which will still display above the background wood image) that matches the positioning and dimensions of the inside of the red wire frame surrounding the player's creature field, permanents field, and weapon/shield field. Set up a similar span for opponent. It should default to being empty.
-Create a bunch of "play field" images that match the dimensions of the spans that were just created. These can really be anything artists want to create, though having some that are large versions of certain card arts, or element-specific things like an element's mark symbol on a field of that element's color would be a good starting point.
-Code the ability to both create reward codes that unlock a specific image and to unlock specific images in-game.
-Tie a bunch of quests (and/or achievements) to in-game game field unlocks. Have a bunch of other ones that can be given away via reward codes as prizes or whatever.
-Add a "game fields" button to the settings/logout area (or insert it as a sub-button within the settings display) that lists an account's unlocked game fields and allows them to select an active game field.
-Include the active game field when submitting arena decks or participating in PvP.
-Once we have enough game fields, start giving some of the AI decks (probably demigods first) custom game fields.
For those of you who play hearthstone, this is basically the card backs system except with play fields instead of card backs. Coding effort seems like it should be relatively minimal.
edit: also same thing except for "portraits"