No, the megacities are not all set in stone. I mean yes, we’ve developed some in-canon: Atlanta, Moscow, Seattle, Tokyo, and New York City for example. Generally if the population in the 22nd century was large enough where it could profit off the construction (such as the aforementioned) it could be funded and done with. I imagine Vancouver or Calgary would be likely prospects. 

New Anchorage is in Alaska, yes.