First think about this, vulcans and romulans are two sides of the same coin, one embraced logic while the other embrassed their passions. I always saw the two factions as the two extremes of what the humans were possible of being. We can be extremely passionate (not just sexually :p ) and we can also be extremely logical, and most of us tend to sit somewhere in the middle.
The federation formed when vulcan logic tempered human "passions" and lead to a civilization based on peace, coexistence, and "what is best for all."
I imagine if humanity had met a "mentor" race that taught the embracing of passions over logic, ie romulans, that the "federation" would have been more of an empire founded on "the strongest rule" IE if the humans didn't join the romulans they would have been the center of an empire very similar in its creed and values. they wouldn't nescessarily be warlike such as the klingons (after all the romulan empire wasn't truely warlike), but they would be very "selfcentric" focusing on what is best for humanity, not "what is best for all" like the vulcan based federation is.
very good logic you must be a vulcan.
personally I give in to my passions but then I'm Romulan.