Call me crazy, but my sister's a huge PeeWee Herman fan, and we discussed both shows, and how Paul Reubens worked with a lot of people from MLP:FiM..
I think it would be pretty damn hilarious to have him on the show. (perhaps a as a certain grown-up Pheonix? :3) My sister showed me all his movies and a couple of episodes from his show... And Paul Reubens himself is a pretty good voice actor too, he was even on Chowder! I also met the guy, and I have a lot of respect for him despite the whole thing that happened with him. He really gave children's television a great running start, and plenty of things he came up with on PeeWee's Playhouse are reused in children's television today.
Going a bit off-topic but man, tha'd be awesome.
Tim Curry... tha'd be great too. XD