Ummm, although I don't know why you would compare this series to Pokemon, I'll go along with it. No, they really don't have the animal themes in all the worlds. About the usual pattern, the whole point is to visit different worlds and get these feathers, so aside from adding some interesting tidibits, then yes, that could be called a pattern. Of course, if you're a fan of CLAMP, then you would recognize many of these worlds and characters, often in different settings, but still the same. You may think that the pattern would get boring, but let me assure you that so far it has me interested, and the fact that each of the main characters had to give up something most dear to them also makes this better.
Let me also tell you that you miss out on a lot of good manga by not reading stuff that hasn't been made into an anime. There are a lot of good ones on the market, and to me when there's an anime linked to it, I usually don't get it. This is simply because either the anime ruins it for the manga, or the manga ruins it for the anime unless they have completely different events in them.