Wind of Change

Radical: the attack and defense of the unit casting Dimension Door drops to 1 until the end of the day (mages are in profit!).
less radical: the defense of the unit casting Dimension Door drops to 1 until the end of the day.

"until the end of the day" doesn't fit here; the blue color will clearly be at an advantage, perhaps "until the start of the next turn" should be considered instead. And in general, all this is suitable for battles with neutrals, but it wouldn't hinder taking opponent cities and using Town Portal to reach an unreachable zone.