Just a weird, theory. Isshin has used Getsuga Tenshō without (from the looks of it) releasing his sword. I think (but not sure) that we have seen one or two demonstrations of a shikai attack in a sealed form. Is it possible its a technique of the sword, and not an actual special attack, nearly unique to the user? The only reason I suggest this is because from what we semi-know, is that Isshin has what seems to be a fire[y]ish type zanpakuto.
It makes more sense in my head, but yeah. Is it "possible" if so, could Ichigo get another attack besides Getsuga Tenshō like Isshin has?