If it ever were to happen, I think Brexit is a really good road map for what not to do.
It might be possible to remain part of the US trading block and use the Dollar while keeping open borders.
Basically New England becomes the equivalent of a Euro using EU member state.
Some of that wouldn't be in New England's control, of course, and there's some risks to not controlling your own currency, but as an idea of a more EU-like US with greater 'sub-federal' autonomy it's not ludicrous.
This way New England gains far more sovereignty without severing all ties.
Fair, though the larger economic entity gets to set most of the terms of participation. Part of the Brexit headache was that the EU was perfectly content to let the now-more-sovereign UK stay in its common market...if it accepted the rules of that market. Sovereign New England would presumably have to make the same kind of choice between imposed economic regulation or risking worse trading agreements with the remaining US...and unlike the UK New England has no meaningful experience as an economic entity outside the US federal structure for centuries, if ever.
I'd also argue that the SNP have definitely talked about how an independent Scotland would run.
Holyrood is already far more devolved than any US state and rejoining the EU and adopting the Euro would be a given.
I was being somewhat flippant, but mostly referring to the kind of not-necessarily-easy trade-offs independence would bring that the SNP doesn't love to discuss when making a public case for independence. (That wasn't a criticism as such, obviously they don't want people thinking there's risk of upheaval to trade or travel or any number of other things, because that'd just incline more people to vote for the status quo.)
I think the biggest lesson would be in setting out any parameters for a referendum. 50% plus 1 seems like a recipe for disaster.
Yeah, that's definitely the best lesson of the Brexit debacle, followed closely by the complete lack of terms on what Brexit was going to look like. It might be viable to run a two-stage process, maybe with a simple majority or maybe with a higher threshold to commission some body (functionally akin to the Continental Congress way back when) to explore and negotiate the terms of a presumptive-but-not-guaranteed independent New England, come up with our version of the Brexit deal, and then have a vote, with both a high threshold and knowing the terms of settlement, so that it's clear that there's strong support for the chosen outcome, rather than allowing a pack of promises and lies to barely swing the day.