Well seeing as I first attempted the TOS excelsior idea, I figured I re-approach the situation from a somewhat logical direction.
First: The ship would have to be either the conceptual/proposed prototype during late TOS or be a spaceframe that was kinda established to some degree already.
Second: The ship cannot just be a Direct retro-ization of the Excelsior, but rather a ship that was more in-line to the established fleet ethic(meaning it couldn't look so different, as it would then not be able to surprise the crew of the Enterprise upon seeing her in ST3).
Third: Establish a design lineage that again fits within the established fleet ethic, but still invokes some sense of originality.
Fourth: The ship's spaceframe was probably a rare thing, possibly relegated to specific roles, as the Constitution class were the real workhorses of the fleet(not to say that their were no other spaceframes in numbers, but Connies have been shown to be numerous, though they probably were not the most made designs, rather they were deepspace explorers that were not to come home anytime soon once launched)
Lastly: It the ship has to look cool (LOL)
Anyway, here she is, my interpretation of the TOS Predecessor of the Excelsior-Class... USS Endurance, Exemplar-Class starship:
Enjoy