I like the Doomsday idea. +10% taxation; -5% or -10% happiness, or whatever it's termed.
Would it be for individuals, or the faction leader?
Shouldn't it apply to all controlled provinces?
Maybe it could be offered as an option, as they do in Anno Domini? You can gain the tax revenue but at the cost of a greater chance of unrest. Actually, I don't think it would be that difficult to script in.
Siculus