Simple question for the start: What Princess of the show would you like to see as a villain?
I guess, many will say Nightmare Moon, and I can agree with them. Nightmare was a freaking awesome villain (especially in the season five finale), and many fans love her and Luna.
However I guess appart from Luna I'd love to see Celestia as a villain. I know that she cares much for her people so I dare to say that she might do something really drastic in order to save them. Celestia is all about believes and I am sure she could twist her believes, if she is forced to do so. Also she would probably look awesome.