Also, I'd have the Romulans already be known to the major Galactic Players (The Vulcans, Andorians, Orions, etc). They wouldn't be the "Out-of-nowhere" enemy that fanon made them out to be.
Um, this is not fanon. In "Balance of Terror," Spock seems genuinely surprised to see his father's doppelganger on a Romulan ship. And when he was recounting the Romulan War, he made it clear that the adversaries never had face-to-face contact.
Agreed. However, I can't help wonder whether he was surprised more because the Romulan was clearly related to Vulcans, or because he looked like his dad ;-)
But to reiterate what JiNX said, from "Balance of Terror" --
Spock: Referring to the map on your screens, you will note beyond the moving position of our vessel, a line of Earth outpost stations. Constructed on asteroids, they monitor the Neutral Zone established by treaty after the Earth-Romulan conflict a century ago. As you may recall from your histories, this conflict was fought, by our standards today, with primitive atomic weapons and in primitive space vessels Which allowed no quarter, no captives. Nor was there even ship-to-ship visual communication. Therefore, no human, Romulan, or ally has ever seen the other.
Earth believes the Romulans to be warlike, cruel, treacherous, and only the Romulans know what they think of Earth. The treaty, set by sub-space radio, established this Neutral Zone, entry into which by either side, would constitute an act of war. The treaty has been unbroken since that time, Captain.
Canon = What was seen onscreen in any Star Trek episode or film.
Fanon = What fans think
The no-visual thing is canon.
Granted, TOS was made in the 60's, and I'll bet that Gene & Co. couldn't predict that 40 years later someone would have to deal with TOS canon.