
Protected by the Sun, The Guardian of Life. The Planet of Earth... Sailor...Err....
Naoko established that only women could be senshi while giving Mamoru a Sailor Crystal...in spite of Mamoru being bound by his previous bodily form.
There is no continuity in which he could ever be a Sailor Senshi.
What I do find intriguing is that Mamoru, like Usagi and Chibiusa doesn't have a miniature Sailor Power Guardian, or for that matter, power upgrades.
Speaking of Chibiusa...given her lack of relationship to ChibiChibi, and by extension Cosmos...theoretically, there is a continuity in which Chibiusa is not born to Mamoru and Usagi in the future.
Now that she has an independent Crystal, she can theoretically be born in a different time, with Usagi, potentially, having a child as did the original Queen Serenity, where they share a common Sailor Crystal and life force. Usagi choosing to retire within the Cauldron as did her mother.
Perhaps this is the when and where Princess Lady Serenity is from?