Today I got to thinking whether Nehelenia might very well have been the original Queen of the Moon under Chaos who was banished to the Moon's interior/dark side.
This came to mind on the basis of her having been present at the time prior to Usagi's birth wherein she seems to know Queen Serenity from a time before Silver Millenium. Likewise, the arc itself is alternatively called the Dead Moon Arc which gives credence to the possibility that she may have reigned over a more malevolent Moon Kingdom in relation to the Earth, whereby mortal men lived in fear of the inhabitants of the Moon (which would explain the uprising during Silver Millennium. Metallia sought to get revenge for Nehelenia being banished and sealed away.).
However, what struck me was that Nehelenia was able to enter Elysium undetected. Some part of me suspects that at one time or another she had a unique relationship to the people of Earth which so allowed her that sort of access. After all, how did she manage to awaken the Amazon senshi from the future, or know of their presence?
Barring her having been the previous ruler of the Moon, it would be nice to learn that she was the former advisor to Queen Serenity from a race of beings who lived in a world of mirrors. This would explain why she was as skilled in creating glamours and other illusions. I also like the idea of her alternatively being a relative of Mamoru's, but that would only further complicate things.