My take in history is that the American South would have had to deal with slavery before 1900. Maybe it would have been better for them to deal with it voluntarily--even though another generation of slaves would have been the result.

Often our national and provincial/state borders do not make much sense---either back then or today. They were mostly land grabs by local politicians somehow caught up in the "conquer or be conquered" mentality of those times. I can think of several practical border changes within Canada, which might include a few new provinces.

Watersheds would make great boundaries.

If we are to allow secession, then various parts of the provinces and states should be also allowed to leave. I think the eastern half of Washington state would prefer to be in Idaho.