As a Lutheran, I'll chime in.
Regarding the question if Luther ever considered the possibility of a new nation being founded on Protestantism - the answer is, no. At the time, the New World was hardly more than a curiosity to most Europeans - including Luther - and the notion that independent nations could arise there wasn't on anyone's mind at the time. There was just barely ideas of colonizing the new lands to the west at the time. If Luther had any nationalistic ideas, they would have been concerning his own German state of Saxony first, and the greater geographic, linguistic, and cultural expression of Germany secondly (as a united country of Germany wouldn't exist beyond the rule of the Holy Roman Empire till centuries later).
As for America having been founded on Protestant ideals - I think that argument holds much water. At the very least, Protestant England had began colonizing North America in part as a response to their Catholic rivals, France and Spain, in order to grab up as much land as possible before losing it to their theological rivals.
And while Roman Catholicism has grown considerably in America, for most of it's history, America has been a largely Protestant nation steeped in Protestant ideals and beliefs.
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-Bill, otherwise known as Kraichgauer