If Gene, god rest his soul, wanted a TOS enemy to turn into a good guy one would think that the Romulans would have been that faction since there did appear to be a noble element to them.
One, I'm not sure the name "Romulan" is as recognizable outside Trek fandom as is "Klingon," so the "message" of an enemy becoming a friend wouldn't have resonated as far with Romulans making the switch.
Two, I think there's a pretty good argument to be made that the Klingons were a "bigger" enemy than the Romulans, even if we confine ourselves to the series. Klingons appeared as adversaries in about a half dozen episodes, while Romulans appeared twice. If you throw in the movies, which were much fresher in the audience's mind when TNG
hit the air, the balance tips even further in the Klingons' favor. Did a Romulan even appear on the big screen before Star Trek V
? Even if one did and I'm forgetting, the Klingons loomed larger in the film series.
So, while what you're saying arguably makes more sense from a story-logic perspective, it just wouldn't have the same impact for the Romulans to make peace.
Finally, from a purely visual standpoint, a Klingon on the Enterprise
bridge is more striking than a Romulan, especially after the Klingon makeup change. A Romulan on the bridge might have evoked Spock a little too much.