This is another one of those "How can we be unique" things.
In my Minesweeper Flags online game, I think that replays is probably one of the best features that I have. (Together with the huge statistics database and the possibility to observe other people playing in real time)
Should we support full game replays in our TCG?
If yes, should players have the ability to watch other people's games or just their own?
If yes, then we need to remember during the implementation that a played game should be serializable somehow. Either using JSON, XML or as pure text.