I was just looking at Board Game Geek, and got a vague idea for a persistent CCG that could be played as a browser MMO. You’d control a city in which you can place structure cards. These cards will generate resources every turn. Every day, you’ll draw a new hand of cards, that can be placed using these resources. Some of these cards will be additional buildings (which can also have their own effects), and others will be unit cards, representing your military.
You can take as many turns as you want in a day, but will only get new cards once per day. This avoids the problem of players getting frustrated at only being able to play a few minutes a day, but prevents the game from being too skewed in favor of players who have no lives (like an MMO).
The main game screen will allow drag & drop of all the cards onto a grid abstractly representing your city. This will allow you to lay it out in an RTS-like strategic grid, which will affect siege combat. When attacking another player’s city, combat will be procedurally determined based on your troop formations and their city’s layout, thereby allowing a city to defend itself even if its owner is not online.