One of the aftereffects I'm really eager to see is how having a lot more hostile southern neighbour will affect Canadian politicians position towards defence spendings into the late 19th and early 20th century.
Its not like in reality it was all nice and well but there also wasn't a war that had just ended less than a decade before the Confedation was signed...
I don't see Ottawa allowing the defence budget falling into decay as much as it did in our timeline. I also have doubts the militia solution that was historically adopted would really be enough. But at the same time, I can't figure out where money for a 'true' peacetime standing army could come from.
One thing is sure, I'll have some more old Victorian-Area forts to visit in my area! Hometown is barely two hours from Riviere-du-Loup and the town of Temiscouata. There's already a restored fort from the Arostook War over there and I imagine a couple more would've been built in the case of a war in the 1860s!
There's going to be a very different attitude in Canada regarding defence spending in the aftermath of the war. Though that will also cause some friction as the leader of the Great Coalition was notoriously stingy with defence spending and didn't much care for soldiering. Then one of our viewpoint characters, a certain Denison, will play a very large roll in both post-war military issues and military politics. The issue of paying for it all will be...contentious to say the least.
Though you're right, many forts are going to be refurbished and not allowed to fall into disrepair TTL! Though many of the current fortifications in the New Brunswick/Maine frontier are currently timber and earthwork forts garrisoned by the militia and a few British garrison batteries.
For all the idea of revanchist America, simple economics make that impossible for us against GB. Between 1881 - 1914, 48% of all foreign investment into the American industrial sector came from Britain. Unless America wants to be massively economically behind than otl, they cannot afford to alienate UK's investment post-war. Similarly, Britain can't afford to lose the American markets. Simple reality of economics.
There will be people on both sides who have been financially ruined by the war. There was millions of British pounds (I've seen the exact number but for the life of me cannot find where I have written it down) tied up in American railroads securities, much of which is now gone. Though TTL there were suggestions (and I've seen people who have suggested this in real life as a solution) that the Union simply seize British properties and assets to help the war, or launch some kind of
Embargo Act against British goods. That these measures would be horrendously self-defeating is something I think Lincoln and his cabinet would realize, but even so, millions of dollars in property and shipping has been lost and the lack of trade is crippling many industries.
One of the only men currently turning a profit is a certain Benjamin Butler who is indirectly running a blockade cartel from Boston.
In the post war world there will be a lot of realization of just how much money stands to be lost if there's another war. There may be strategic considerations that mean the losses may be necessary, but then again, there also might not. The late 19th century is going to be an interesting time in the Chinese sense!
I find it probable that there will be more bad blood between the commoners of the USA and the British Empire, but the higher level political and financial leaders will come to see the war as an unfortunate result of a small handful of men's incompetence and arrogance. Lincoln is unlikely to be as well-regarded as OTL in this timeline and Seward is likely to be demonized by a substantial chunk of the populace and/or intelligentsia for years to come.
There will be
many interpretations. It will depend on where you're from regionally (New York for instance will have a very different attitude than say, Michigan) while in Britain it will probably be seen as a sign of Yankee foolishness until the end of the Victorian period. In Canada you'll have it as another sign you can't trust Washington, and much more besides.
There will be bad blood north and south of the US/Canadian border for quite some time. There will also be a lot of angry looks across the Atlantic.