Bingowings said:

But whatever is done it needs to be subtle (a full body floating apparition blocking all of Palpatine's blasts would look really corny and without new footage very fake, the PT has enough heads tacked on to CGI bodies as it is).

Actually, if the Yoda/Palpatine fight is cut from RotS, some of Yoda's defensive posturing could be used. I'm not sure what would work for Obiwan.

But yes, the key word is subtle. As though the force lighting is the only thing illuminating them until poof! they're defeated and disappear.

Edit: This would be cool.