I wouldn't make any assumptions yet on the file format, it may very well be NIF given their extensive experience in using it.
I can speculate
imo. It's either going to be HKB/HKX. this is the havok file format for assets and animation. Now seeing Havok Behavior is used, which means they are probably using the havok behavior tool kit for animation set up. This would make a lot of sense.
Good news about that is that you can DL havok developer tools for free.
however, bethesda is likely using a custom code of havok, much like they have done since oblivion. Bethesda's havok blocks always started with BHK, which I always thought stood for bethesda havok, And as such the vanilla havok tool set isn't going to export files with the data structured in the same way if they need custom code. There could be a simple cmd line proggy that fixerates the data structure though. Now that would be the best all round solution that satisfies both Beths need for tweaking the code, and ease that modders can get assets into a usable format, while still having use of full developer tools for exporting.
Or they will use a new bethesda format. Something starting with B I reckon. While that sounds cool, there maybe an issue with them releasing all sorts of modified code for havok tools and what not
worst case is still no tools. But at least if they went a custom hkb format. we'd have a rather large start to getting working assets made by the community.