I would suggest to make the revenge system similar to the war system with the following rules:

- Max one revenge per day per attacker.
One revenge per day but there is no need to wait for the defender to go offline or wait until his shield is expired.

- Revenge is done on the war base of the defender and not on the home village.
You want to simulate a sort of war attack so defend with your war base.

- How much loot can be stolen is probably a complex algorithm but the idea is that a small percentage is deducted from all the clan members and not just from the defender. There should of course be a max per day on this. There should also be good bonus because the idea is that people attack in a revenge with a max army similar to a clan war attack.

- Revenge earns you a small amount of clan war XP.
This can e.g. be 1XP for each star that you get unless the TH of the defender is lower then your own TH. In that case no XP is rewarded.

The whole idea is that you can sort of practice war attacks while you are not in a war. Let me know what you think.