Battle Summary/History
Sometimes visual effects are so quick that I don't know what happened. It would be nice to be able to review a transcript of the battle after it is done so I can see what actually happened in places where I was confused without slowing down the actual battle (and sometimes I can't see the card any more once the effect takes place, such as if it was exiled). While a text log would suffice, with the level of visual polish already presented, I would recommend allowing users to step (forward/backward, and maybe navigate a timeline with turn numbers) through past battles, with the ability to see all cards in all locations, even those not visible to them during the battle. This should be available for at leas the last battle, with the option to go back to view (sometimes I accidentally exit to the main menu when I wanted to review the battlefield first, and it would be nice to be able to go back after cooling down following a devastating defeat), download a battle to their device (for bug reporting, sharing with friends, or personal archiving), load a previously saved battle for viewing (unless the download format is human-readable, not being able to do so would limit the functionality of downloads to bug reporting), and if file size for a battle is not excessive, perhaps a history of the past week's or user saved battles could also be kept on the server. One potential additional feature that might require more discretion would be the option to copy the opponent's deck; this would require acquiring any cards not already owned (the deck might be missing cards), which could increase demand for acquiring new cards, but might adversely affect players who spend time carefully curating custom decks.