Notification for Online/Shielded villages or grey out revenge button
An icon or simply graying out the revenge button if person is online/shielded.
I hate going through my attackers and finding almost all of them online/shielded after I check their base to see if it is worth it. This would save me a lot of time!
Solving privacy issue with revenge
Since the greyed out revenge button would be a problem for privacy issues, a solution would be to make the defense log similar to the scouting function of clan wars. In clan wars you can scroll from base to base without having to return to the war map each time. I believe that by adding this already developed function to the defense log would not only make the searching aspect run much smoother, but would avoid the privacy issue altogether.
A second approach could be to add the revenge option to the screen while a player is visiting a base from his defense log.
I believe that implementing either of these aspects would increase a player's likelihood of revenging their opponent, without the worry of a privacy violation.