During the American revolutionary war I believe that the mainland area east of Maine and north of the Nova Scotia peninsula (that became the Canadian province of New Brunswick) was under the administration of the British Nova Scotia colony.
There have been discussion in the past about how British retention of Nova Scotia was overdetermined, because except for an isthmus, it is practically an island and Halifax was constantly a major British naval base continuously occupied. This is so even though a lot of the Nova Scotia population sympathized with New Englanders.
However, those objections do not seem to apply as much to the land that after the war became New Brunswick. Could that land have plausibly gone to the Americans? Perhaps as a federal "northeast territory", an extension of Massachussetts Maine district, or a state of "Nova Scotia" wherein the mainland part goes to the USA while the peninsula across the isthmus stays part of BNA?
Indeed, why did this area *not* become part of the United States at the end of the war, even in OTL.
In OTL Maine was a hinterland dependency of Massachusetts and New Brunswick was a hinterland dependency of Nova Scotia. But Maine and New Brunswick's borders with each other are much longer than their respective borders with Massachussetts (none by land) and Nova Scotia (a small isthmus).
Maine-New Brunswick seems like a more geographically logical unit or package deal rather than either of Massachussetts-Maine or Nova Scotia-New Brunswick.
This aspect of the border makes it seem to me that an American New Brunswick is far easier than an American Nova Scotia.
Of course the same question works in reverse. It is even more remarkable that Maine ended up under US possession and did not remain under British North America.