Romulans would have been great - but I'd like to have seen Archer and Co. essentially create the Romulan threat - when Enterprise first aired, there was one line of dialogue from an otherwise mediocre film that stirred my interest in seeing the Romulans -
"Warp drive turned the Romulans from a bunch of thugs into a galactic empire."
It would have been neat for them to go "Well, the Orions(/Kzinti/Insert alien threat here) are a real huge pain in our ass, but we've found these proto-Vulcan dudes with industrial capacity and if we give them warp drive, they'll help us."
Now, the visual contact thing does throw a wrench in it - but in that kind of scenario, I'd imagine it would be such a massive upf**k that Starfleet would seal the records and tell the crew of the NX-01 to keep their pie holes shut.