Sidious IS Plagueis
OK, this might seem a little farfetched, but bear with me. Sidious said that only one had actually achieved cheating death. Now get this…perhaps he used the dark side of the Force to make his appearance remain moderately youthful and prolong his life. Then, when Mace uses his force lightning against him, it shows him as he should truly look, his skin old and decadent due to years of abusing the force to do his will.
That might seem like a pretty wild idea, but I wouldn’t put it past me. I’ll elaborate on it a bit more later.