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.
Basically, they'd be these shady arrogant Empire that have built up their Empire on Slave Race Soldiers (Remans, Suliban) and demand tribute from smaller weaker worlds as Protection Money (but they do actually bother to protect them from space piracy, so while they are jerks and bullies they'll honor their agreement. Don't want to give away the goose that lays the golden eggs, right?) while using agents to perpetuate discord and negativity among those who would rival them.
Humans, having been sheltered for as long as they have, were under their radar and as such might one day become the rival the Romulans feared (they're building up their numbers for war one day, but need to keep everyone else occupied until then).
Also, the Romulans have extra-special "Vulcan" Emissaries they use only in rare cases. Like when they send some to Earth or wherever the Galactic Embassy is to inform them of where they can and can't go.