My prediction is that they will release it compatible with 3ds, but with better frame rate on N3DS and a secret cave with a rare pokemon acessible only to people with a pikachu amiibo.

I'm not entirely sure there is even going to be a third version. Sure, there's a precedent for it, but Black and White having direct sequels rather than a third version makes me wonder if Nintendo's shifting its strategy for that series. Assuming there is a third version, like other posters have said, it will probably just have exclusive features for New 3DS while being compatible with the current one. Nintendo's not going to ignore a 40M userbase.

Well, BW/BW2 also had an actual story to them.

XY were much more back to basics with the more generic story form of: You. Special. Go. Become. Champion.

The better question is, "Will ORAS utilize the extra power?"

Pokemon Z will be playable on the old 3DS. Even Black 2 and White 2 could be played on the original DS from 2004. What they may do, as others have mentioned, is have better frame rates on the New 3DS, while it still can be played on the old 3DS. Think of it like Gold and Silver. They were labeled as GameBoy Color games, but could be played on the original GameBoy. You just got the bonus of colors by using a GameBoy Color.

The answer is, "No."

Says who? Nintendo has said that there will be games that use the extra power of the New 3DS without being exclusive, and the new 3DS comes out before OR AS. And there's a precident for thid kind of thing with Pokemon Yellow and Gold/Silver.

Pokemon is always late to adopt the new technology.  Although OR/AS are coming out after the New 3DS, that's only true with Japan, and they were announced long ago.  They also would have likely revealed that in the Japanese Nintendo Direct.

Or they're waiting for next month's coro coro to reveal it.

It's not impossible, I just think it's very unlikely.