Mmn, not sure if it counts since it's a show and, more importantly, was abandoned before they even got any audio animatronics in (AFAIK), but the English version of "Meet the World" that was meant to be put in the show building in the Japan Pavillion. I saw a video of the Japanese version online and frankly it was much more enjoyable than, say, Impressions de France or the old Canada show, and about as interesting as Reflections of China and the current O Canada- and that's just for the "content". The fact it had audio-animatronics as well boosts it up quite a bit, if you ask me. I understand why they didn't put it in, but the World Showcase is meant to be about embracing worldwide unity and not "remember that time your country did that bad thing?". And hey, just one pavilion over, the bombing of Pearl Harbor is blatantly mentioned so...
Eh. A lost cause right? I guess... It just feels so strange that Japan is a "shorter" pavilion than most of the other WS pavilions. It just doesn't have length like Germany and the UK or depth(? I think that's the term, as the only other I can think of is height and it sure isn't that) like Morocco and Italy.
Anyway, out of things that actually existed, probably Cranium Command, the old Imagination or Mr. Toad's Wild Ride since I never got to go on either of them- the one time I went when they were open (I was six, I believe), we just didn't do them. They sound really fun too. Better than the current Imagination! anyway, heh heh.
I asked a cast member last year (first time back in 30 years) where the 20,000 Leagues Under the Sea Submarine Voyage ride was located. I loved that ride as a kid. The CM (who appeared to be about 14 years old), without batting an eye, told me where it used to be. I teared up.
I know I already chimed in on this one but I had some time to think on this. I'd like to see the Magic Kingdom Skyway come back. It was the worlds first Skyride with a 90° bend between the two stations. I'd also like to see Alien Encounter return, that really was an incredible experience. Stitch's Great Escape doesn't even come close to what AE was. It's also in desperate need of a refurb, just look at the padding ob the over-the-shoulder restrains. For an honorable mention, I'd like to see the Swan Boats return to the MK.
To answer the OP's question (only picking one), it would have to be the Irons version of SSE. I just can't get cozy with the new one...and I've TRIED repeatedly....I spend the entire descent envisioning what used to be in so and so place and trying desperately to cover the monitor with my arm.
Since most of the big ones have already been mentioned, I'm going to go with something a little odd. The Fort Wilderness Railroad.
At WDW, I would like that they brought back the Body Wars area. Fir me, it is wasted land in the park, as they need more attractions
They should take the Wonders of Life Pavilion and use it for a whole new themed area. Have it be something with Technology where they can have different sponsors come in and showcase different ideas in our technological age. I'm not sure what could be done in the way of actual rides, but the idea is a start. Its a good thing the Festival Center is there now, which would prevent Disney from using it to help promote Iron Man 3, which would quickly turn into an obvious promotional stunt which would run out of gas. The idea itself for it to work would need to stand on its own. I know there is the E3 expo or something, but not everyone can go to that.
I hardly go in there, so no wonder I forgot. :p
Journey Into Imagination