When it comes to solid ideas, how easily they are exploited is a big factor.
As for the idea, here are my thoughts:
Having a small defensive bonus for the lower leagues wouldn't really make a difference. There is no need to implement that imo.
Having a defensive bonus up high is not really needed either. If you have a base that can defend up in champs/titans, you probably don't care about the loot anyways.
The people there that do farm there usually have their town hall out to protect their loot, or weak bases that can't defend anyways. The effect of a defensive bonus would end up with the players that don't really need it.
(The attack bonus helps cover costs of the army. A defensive bonus doesn't really have much of a functionality)
The masters leagues are a popular place to farm nowadays, so a defensive bonus there would be exploited faster than lightning strikes.
Thanks for reading. A nice day to you :)
