I'd be remiss if I didn't mention that the whole premise is pretty tricky to actually get going. In most cases, you'd have to create ATL revolutionary movements in those regions out of thin air. Ironically, even though they were at odds with the colonies, the Quebecois were most inclined to go along with any revolt, out of all the "Canadian" (in the modern sense) colonies. That is to say: there actually were some Quebecois revolutionaries. Presumably because they figured that independence would be an option.
Anyway, supposing this all happens because of some reason, the results are pretty interesting. Losing the Maritimes (Halifax!) and Quebec (Montréal!) means the Brits have about zero reasons left to hold on to Labrador, Newfoundland and Rupert's Land. With no decent ports, those regions are just not going to be worth exploiting. Presumably, they'll offload these regions onto the USA, in return for great peace terms. In practice, that will mean that the USA will promise not to be dickish to ex-Loyalists, will enter into favourable(-to-Britain) trade deals with Britain (as in: private British interests will maintain all their holdings in North America and will not be hindered in their economic undertakings).
It's even possible that the USA might be compelled to explicitly agree to not side against Britain in any European conflicts. (Washington, Franklin et al. wanted to stay out of Euro-wars anyway, and Britain would want to avoid a hostile North America in case of an Atlantic naval war with France -- primarily because such a hostile America could provide harbour to French ships.)
This means
better Anglo-American relations, ironically. You have a peace deal where the USA gets more than it ever dreamed of, where good relations with Britain become a fait accompli right away, and which leaves way more loyalists within the USA. Those loyalists will vote, and they'll vote for the pro-British factions. (So... Federalists will benefit.) There will be substantially more trade with Britain as well, so Jefferson can basically suck it: the Francophile faction won't ever get in power here.
Not that relations with France will initially be bad. Quebec will get its independence, and may even try to get annexed by France. (I expect France will say no. Quebec would be a fucking money-sink, and France infamously
had no money at this junction.) But we must recall, Quebec was pretty conservative. So ironically, when/if the French revolution happens, Quebec will tend towards the monarchist side. So both the USA and Quebec will be generally aligned with Britain, and against France.
Just to clarify, Quebec will NOT be the area covered in the Quebec Act, of course. Quebec will have the southern border of pre-1774 Quebec. Most likely, it'll keep its direct Western gains, though. So it'll be bordering Rupert's Land. The area South of Quebec and Rupert's Land but North of the Great Lakes will become the State of Canada. To visualise the situation, see this map:
The USA in general is now 'pointed' North-West. The Federal Capital will almost inevitably become New York. Southern cities can forget about that, here. (Probably no debt assumption, either. Virginia won't fail to notice that it's definitely playing second -- of not third -- fiddle now, so it's not going to play ball.)
One thing the South will want is "
more South". It happens that the Federalists (the party of "standing armies are cool, actually") agreed in OTL. And in this ATL, there are way more Federalists, and there's probably a stronger navy and a bit more of an army. So once Britain and France start fighting, there will be an opportunity for land-grabbing. I'm assuming that a French revolution still happens, but even if not, Anglo-French conflict is still extremely likely. Spain is tied to France. Spain has land next door to the USA. The USA will side with Britain. So... land-grab time.
I expect the USA to gain Louisiana and Florida not by purchase and diplomacy, but by walking in and claiming the lot. Depending on just how successful the Federalist push for a real military ended up being, there may also be some Caribbean real estate getting annexed. That's more of a long shot, though.
This will probably be the last real land-grab, though. After this, it'll become too obvious that further expansion will favour the slave power, which the North doesn't like. And there is more North in this ATL. Since the USA basically has all of Oregon Country already, without it ever being an issue, gaining more land from Mexico will be relatively uninteresting. Also: Federalist-style policies will be more popular, and those favour industry over agrarianism. This USA will have land enough. It's economic expansion will be more of an urbanisation than a major land-grab like the Mexican Purchase. I don't think even Texas will be tempting enough. (
Especially not if the slave power already got some Caribbean slave states earlier.)
Regarding slavery: I expect the North will be able (and willing) to force it to be formally contained much earlier. Ohio Country will be free soil, no doubt. West of the Mississippi, they may just go back to the Mason-Dixon line, and extend that to the USA's Western border. With further expansion off the table, that'll essentially "solve" the slavery issue for a long time. There will be a bloc of slave states, but they won't be expanding, and they'll never get anything like the Taney Court, so no moving slaves into (or even through) free states.
Eventually, there will still be a clash over the issue. The proximate causes will be different, though.
