And why do you assume that Pein will be able to control Natural energy? Jiraiya studied the technique for years and still deformed.

Controlling natural energy is sage mode. Unless you think that being "Sage of the Six Paths" means Pein knows Senjutsu, or how to control natural energy, if he absorbs natural energy, he is screwed.

This isn't as implausible as some make it out to be. It isn't going to happen though. Oda could pull it off. I doubt Kishimoto can.