Thing is.. plague wind is black.. black has always had a harder time of destroying everything.. white has wrath, winds of wrath, akroma's vengenance, wave of reconing and a few others that all cost 4-8.
Besides with Isao, you'd need to not only splash green.. but be able to pay 4 white in the cost... so for
4WWW+2G= 10 mana for a wrath and 1 creature..
10 mana is usually indicative of an infinite combo. Plus destroying all creatures but removing non samurai would mean the card would be restricted to a black and white samurai deck with reanimating..