Since no one else has mentioned them, I'd like to nominate the Vissians for membership. I have them as part of the first group of worlds to join right after the founding of the Federation, along with Draylax & Denobula (who I have as a founding race of Starfleet but NOT the Federation).
The Vissians are a possibility. We've seen a Vissian Starfleet officer in SCE
(circa 2376), and they're close enough to the Federation core worlds (Brave the Storm
places Vissian space right next to Neethian territory, and Neethia is included in the Star Charts
). I personally prefer them as a friendly, allied power rather than a member, though. Not all peaceful civilizations will want to join, after all.
I also assume that they would need to give cogenitors equal legal status with the majority population in order to meet the criteria for membership, and I don't see their perspective on that issue changing too quickly. Maybe by the 24th Century, but I don't see it as likely by the late 22nd, personally. And their technology is actually superior to the UFP's in many ways, so they wouldn't be enticed to join by the promise of goodies, I'm guessing.