Even the writers of DS9, which everyone seems to put on a high pedestal, just made it up as they went along. They had absolutely no plan either, no idea what would develop from a premise used in the first episode. They just did a better job of covering their tracks when arcs were abandoned. Bajor joining the Federation anyone?
The key to success here would seem to be having no network breathing down your neck. UPN insisted an element like the Temporal Cold War, something as futuristic as TNG/VOY be apart of Enterprise, or else there would've been no show.
I always figured they'd reveal Future Guy as a Romulan at some point. But why a Romulan, other than to satisfy my fanboy wishlist for the race to be important to ENT? Well consider the reasons why he bothers to conceal his appearance. Those he has spoken to like Silik and Archer obviously aren't supposed to positively identify him... Suggesting that if the person's reveal isn't important then his species is. In a line of dialogue from Broken Bow, he states they weren't supposed to involve the Humans or the Vulcans, which implies that he's neither of these. He was basically responsible for genetically altering members of the Kabal, probably like what happened to the Remans. Then there was the stealth technology used by Suliban cell ships. But of course, there weren't going to be further seasons in which this could all be panned out. Since interference from the TCW led to important events like first contact with the Klingons and the Xindi incident, it would've made sense for another faction to be messing with Romulan history too.