What it says on the tin. What if, with an extremely negative turn of events during the war of 1812, secession became the consensus of New England Federalists and it was successfully attempted?
How does the new republic evolve? What does its political and economic life look like 1, 5, 10, 40 years down the road? How are relations between the U.S. and New England? The U.K. and New England?
What long term effects does this have on the balance of power between free and slave states in the U.S.?