This is what I did (I have Alaska too). I've tried a whole Mexico approach to expansion before, but I simply could not make the southern portion of it Yankee. Instead I opted to take the northernmost states that weren't already cored. After that I went North, and was as stubborn as I could be with Canada. I have to say Britain attacks just a little too much, they shouldn't be that adamant to take back Columbia if its gained statehood. Perhaps there should be option to reorganise the territory after the USA reaches up north too, now I have ALL the Canada cores, Rupert's Land and the Northwest territories will stay as it is for the rest of the game. A chance to admit the rest of Quebec, surrender other lands and possibly release a Rupert's Land or rump state Dominion would be excellent, as would an event at some point to clear up the rather bizzare British cores that end up there. If they were replaced with Rupert's Land cores or something and then there were events to push the British out of North America entirely, that'd be awesome.
The same goes for Mexico really. An "All Mexico" approach to Manifest destiny should be an option, but should turn the USA into a pariah and should dramatically extend slave power. Maybe if the USA works hard enough, Mexican could become accepted and the nation could be broken up or assimilated by other powers (maybe the USCA could get Yucatan and Chiapas, a Nahua/Native culture state could be possible to form and America could eat up its neighbours in a historically plausible way ( I can't say I favoured the constant acquire state approach, it gives no flavour to the idea of taking all of Mexico, which was a real political consideration at the time.) Oh, and an event to remove Mexican cores from the USA wouldn't hurt either. Seems a bit odd to have them if the treaty of Guadeloupe-Hidalgo was signed and there are no longer any Mexicans out there.
This is APD, by the way. I relied on a lot of its features.
