User blog comment:RoyalGuard-Elite/Isshin's Shikai/@comment-5147109-20131021203956

It was mentioned in the fight between Ichigo and Ulquiorra that Getsuga Tensho was alot like their Ceros I'm not sure if he is talking about just the black one that Ichigo uses in Bankai or the technique itself, but essentially the technique is nothing more than concentrating Reiatsu along the blade and releasing it with a swing of the sword in which case Getsuga should be usable by anyone since all the Shinigami have Reiatsu, plus all that Cero is is a concentration of Reiatsu released at a certain point, my point to all of this being that I believe that Isshin's Shikai and most likely his Bankai is not so much elemental as focusing his Reiatsu into advanced techniques unique to only him.