And this one on a Rhode Island proposal: https://www.bostonglobe.com/metro/2016/03/11/will-new-englan...

Both explicitly require Massachusetts to go first.

Maine has been considering it for longer (which makes sense, as it's further east), since 2005, according to this story: https://bangordailynews.com/2017/01/19/politics/state-house/... . The proposals there don't seem to be contingent on Massachusetts making the first move.

I couldn't find any references to proposals in Vermont or Connecticut.