Gaia
Gaia… is the personification of the Earth. It’s short, relatively unique, and may pair well with Bellatrix.
I am not an environmentalist, but I think we want a name that commemorates the significance and deeper meaning of the release. 
It is not just a Merge, or where two things meet, or where something ends… we want to convey why…
Uniqueness is helpful when future generations search for the details and stories about the release. A weakness of proposed names like “Paris” and “Serenity” is that when searching for “ethereum paris” or “ethereum serenity” there will be many articles unrelated to this historic release…
If the other contender is Malaspina (or Halocline), it is too complicated and too many syllables: do you want to say Malaspina or Gaia?
For uniqueness, I think city names were a poor choice for EL names. If city names are still desired for future EL releases, I still think this one release can and should be unique and not follow that trend.