The undo functionality is awesome, as is. Just to make it a tad awesomer would be some kind of notice of what kind of "undo" your opponent is requesting. A "big" undo is a request when new information has been revealed. (e.g. "I want to undo my Sea Hag, because he played a Moat".) A "small undo" is when no new information has been revealed. (e.g. "I want to undo my Sea Hag, because I didn't notice that he had a Lighthouse/Champion in play.")
We definitely want to keep the functionality of big undos available. It would just be nice to have the game automatically tell me if information was revealed or not (rather than trying to figure it out from the log). I'm thinking that the dialog box that could turn red if your opponent is requesting a big undo. Just doing that would probably create a cultural norm that we grant small slip-ups, but require you to justify yourself if new information was revealed.
Even better... it could become a game option to automatically grant smalls, and only require permission for bigs. That's how I'd prefer to play. I hate it when I hit Borrow before playing my treasures, and then hope to get permission to make my buys. IRL, that situation would never happen.
Keep up the great work, SI!