It might've been more interesting if he was Deltan or Efrosian or Rhaandarite or one of the dozens of other races that are part of the Federation but haven't been explored or expanded upon. It wouldn't make much sense for the Cardassians to be so friendly in this era only to then swing to the other extreme and become one of the greatest hostile forces the Federation has had to endure for decades, with no explanation as to how/why.
Partly agree here. I'm with you on showcasing Trek aliens that haven't gotten much spotlight or development. I mean, is the Efrosian species name even canon? I would really like to see a live action Caitian crewperson, but I digress.
I disagree when it comes to canceling making Phlox Cardassian due to where the Federation and Cardassian Union are politically in the 24th century. That's two hundred years away. I haven't seen "Chain of Command" in a very long time but didn't Madred once talk about how the Cardassian military took power and I assumed that turned their society more militaristic? Perhaps we could have seen that happen during the course of ENT.
The easiest disagreement here is that Cardassian Phlox is an individual and not reflective of the sum total of his species. He could break a mold, or have issues with how the Union does things or how Cardassians are perceived or stereotyped across the quadrant. Like how the great ENT episode "Judgment" gave us a different side to Klingon culture, Phlox could've done for Cardassians, and fans in the know could feel a sense of foreboding already knowing what's in store between Earth and Cardassia in the centuries ahead.
To the OP, I think the idea of Cardassian Phlox is pretty neat and one I hadn't considered before. Like Bry, I would've made him a member of a less explored species, or just a plain old human. The human adventure had to begin somewhere.
If Phlox had been a Cardassian I don't think it would've made him more popular per se. I don't think Phlox's popularity was an issue for the series, though I could be wrong. Making him Cardassian though might have enticed a few more fans to give ENT a try and to stick with it, but I don't think it would've matter that much. The general public still wouldn't have cared and I doubt how much his inclusion would've helped against studio interference or far too often bland writing and characterization.