I've always loved the idea of a United States that's either a banana republic or a just-emerging regional power. So often in alternate history we either see a huge United States (which engulfs at the very least Canada and at most the entire continent) or the oh-so-typical USA v. CSA split (you, reading this, may even see that map in your mind). I've always had two ideas regarding the size of such a Union in my head: the United States in an alternate Treaty of Paris and a much closer War of Independence is relegated to the East Coast where the borders established are those the Proclamation of 1763 or a United States without the Northwest Ordinance and West Florida.
I'm not sure how plausible such a scenario is (given that it's one where history actively keeps the United States from expanding westward), but I've always loved to toy the idea of a United States which isn't a world power, but rather a nation which, while still important, has nowhere near the say it had at its height/has now as a global superpower.