What if the coastal states seceded from the U.S. and petitioned to join Canada as Canadian provinces?

Well, there is the Constitution. In it, there are no provisions for states leaving. So, I'm guessing it would lead to a civil war. Also, there is a lot of federal property in the coastal states. Especially military bases (e.g. San Diego). That would be a significant complication.

But Canada might actually go for it. Interesting thought experiment. Just the addition of California would be a huge boon to Canada (or any nation...the CA economy is enormous). NY is home to one of the largest financial centers on the planet, which would also be a boon for Canada. And NY does share a border with Canada, so that would -in principle- be easy. Except for the Constitution and civil war thing.

Though maybe civil war would be averted. A LOT of right-wingers want CA to go away. They may not fight it at all (foolishly). Maybe they should be careful what they wish for...they might get it.