Some of the ones considered in OTL by the US founding fathers were "Freedonia" or "Fredonia" (its inhabitants would have been called "Fredes", rhymes with Swedes - no, honestly), "Appalachia", "Alleghenia", and most popularly "Columbia". Of course people forget the USA is really the USNA - all the early documents give it the more rational name of "The United States of North America", but that was soon dropped.
If you go further back in PODs you get things like Virginia or Florida, both of which were used at different points to refer to the whole of North America.