This headcanon elaborates on two ideas; Chibi-Usa’s problem with aging + power, and the ever-elusive Sailor Cosmos.
Chibi-Usa was born with control over both the Silver and Golden Crystals. The reason she was permanently stunted and weak was because her body needed to choose which Crystal to “inherit” before she could progress to puberty, and was in a stalemate trying to choose which one. When Chibi-Usa chose the Silver Crystal for herself, she started to age, and the Golden Crystal became dormant inside her.
However, in the distant future, beyond Crystal Tokyo when she has become the new Queen and Sailor Moon, a villain much like Galaxia appears and takes her Silver Crystal. It is destroyed and sent to the Galaxy Cauldron to repair and be reborn.
Normally Chibi-Usa would die in such a situation. But because she had two Star Seeds, the Golden Crystal shifted forward into the dominant position, replacing the Silver Crystal.
But the Silver Crystal still went to the Galaxy Cauldron and still needed to be reborn. With its original host living, essentially, with a replacement Star Seed, it had nowhere to go, and so drifted off into the ether; to the distant future? To an alternate universe? That’s uncertain. But being so powerful it eventually gained a human form, similar to the Crystal’s original host - Queen Serenity - and with only a vague personality that was a diluted shadow of hers.
Sailor Cosmos is not a person. She is the personification of the Silver Crystal, and that is why she returned to that past; not because of Usagi, but her Crystal.
Meeting Usagi gave her humanity.
This clears up a lot of questions I had wondered ^_^