As the title says, the banker could levy a tax on income from looting.
I hope that it would encourage realms to start seeing looting as a legitimate source of income. Some smaller, poorer realms (OI comes to mind on EC) especially could benefit from essentially living off the lands of others, if they prove competent enough that is.
And well, to be honest, I think it'd be pretty cool if there were to be more raids on top of the "I will destroy you!" kind of wars we usually see.
