New league - new builds?

Can't seem to find any information about it - will there be changes to game mechanics or would 2.2/2.3 builds still be viable in atlas?

We expect some changes, yes, what we heard so far is:
- Channeling spells will use the actual channeling mechanic now, so don't plan around stuff like Incinerate or Pizzablast.
- Cast on crit will be 'substantially reworked', which could mean it will function in an entirely different fashion, or maybe not, another thing that's up in the air.
- We know that devs are 'mildly unhappy' with the way some DoTs effectively double dip, that will probably be changed at some time but we still don't know if it will be this patch or some of the future ones. It will change poison, bleed and incinerate builds substantially when it happens.

And there may be changes and nerfs to individual skills and items, probably little to none on skill tree. That may invalidate certain builds so you better hold off the theorycrafting until we see the patch notes, we usually get the preliminary version a few days ahead so that's plenty of time.

Chris Wilson posted on the subreddit that they're planning to release patch notes on wednesday.

Source: https://www.reddit.com/r/pathofexile/comments/4zrxky/riot_plsi_mean_ggg_please_could_we_get_the_patch/

Oh and I remember Bex confirming that the passive tree shouldn't have any significant changes.

Is there even any new league info? Or is the expansion patch the new league?

It's the Essence league, at the very bottom of the atlas page. You can get a bit more info in that video interview.