I think that a special "45th anniversary opening" should have a mix of new footage and existing footage. Clips can appear on a screen-within-a-screen (don't know what it would be called), which would take care of any full screen/widescreen concerns.
Good clips to include in a montage in the show opening would include: Ernie singing Rubber Duckie, Elmo in Elmo's World, Cookie Monster as Allistar Cookie, Grover serving Mr. Johnson a plate of spaghetti, Zoe, Abby, and Rosita from "Because We're Friends", Snuffy jumping on Big Bird's trampoline, The Count on Beat the Time, Big Bird rollerskating from "Wheels on my Feet", Bert "Doin' the Pigeon", any Telly and Baby Bear clip, any Murray Monster clip, any clip of Cookie Monster eating cookies, any clip of Oscar, Ernie and Bert playing the rhyming game, and any Super Grover clip.