Scott Kingsley Clark posted an update of the Fields API on the Make WordPress Core site. It’s a good summary of what the current API can do and what you can do to help. “Every area needs review,” Scott remarks. The next weekly meeting is Monday, March 16, 2015, 4:00 PM EDT.
There’s a great deal of excitement around this project. Personally, one thing I’d like to see is more visual demos and screenshots so folks can get a better idea of what’s possible. It needs to be a viable plugin for use right now (in lieu of CMB2, ACF, etc) before it’s ready for core passage.