Pay gems to get revenge on shielded opponents
Exactly what I suggested in the title: for the cost of some gems, you get to take revenge on an opponent that is under shield or guard. The loot would be either a) what they raided from you plus a fixed percentage (maybe 10%), or b) whatever is in their storages (whichever is the lesser of the two). Along with this, you would only be able to gem for revenge within a specific timeframe (maybe a week?) and you would have a limited number of gemmed revenges per day (maybe 1-2).
Otherwise just scrap revenge. The revenge button is a massive tease that doesn't do anything under the current system.