Ow! You're making my head hurt with your confusion on the 3rd movie released (Episode VI) and Episode III. First you referenced The Return of the Jedi (Episode VI) as EP III, then subsequently referenced Revenge of the Sith, correctly, as EP III. Please get your references correct. When I initially read your first sentence, I was really confused. Episode III was the darkest of all six movies, and my personal favorite, as it ties everything together.
In Episode VI, the native creatures were originally Wookies, but Lucas et al. thought that kids would be scared of them, causing loss of potential revenues. Additionally, it's easier to make toy Ewoks than toy Wookies.
We finally got to see the Wookie planet in Episode III, but I still would have preferred to have had it appear in its original location in the series, Episode VI. That being said, what's with starting in the middle, then going back to the beginning so many years later (Star Trek also, anyone?!? That latest series just destroyed the entire franchise, IMHO)?