So, for the first of what may be many ideas on the subject, the unexplored Dark Ocean subplot. Specifically, the Hangyomon that reveal themselves to be servants of their undersea god, Dagomon. It's easy to conclude these creatures are supernatural entities of an entirely different caliber than Digimon, nothing more, nothing less, but, what if they were something before?
To begin, this idea was inspired by what Hikari says in this scene:
Hikari has just been stalked by a mysterious creature, and earlier experienced her physical form phasing out--both of these things she implies to have experienced before to the point of familiarity, and that her brother has always been at her side during these times. What's more, her earlier encounter in the hallway, when she sees herself as a child talking to her brother (both at Adventure ages), it implies it's something that's likely gone on for years.
I assume it started occurring some time after the end of Adventure, for reasons that have to do with the creatures' "true" identities.
The creatures' original forms...were the Numemon freed by Hikari and killed at the hands of Mugendramon.
I know, I know. Digimon are reborn when they die in the Digital World. Stay with me here.
Hikari's psychic abilities go largely unexplained in Adventure, leaving room for plenty of speculation about their origins and whereabouts. What we know about it: it's immensely powerful, it's awakened when Hikari is in a moment of grief, and it invigorates others. The key here being that it gives life. It all suddenly started clicking for me when I read Dave the Analyzer's post here--what if Hikari, who had been given these powers of life, was granted them during her brush with death?
With these unique abilities, could she possibly give unique life as well as death to others? In the case of the Numemon--could this newfound, mysterious energy she gave them have influenced the fate of their deaths and rebirths?
It seems like a fitting, if tragic, fate for Hikari's great power to also be capable of bringing great destruction--the creatures of light becoming creatures of darkness. And weren't the Dark Ocean creatures suspiciously similar to the Numemon?
They worked again as servants for an evil overlord (this time, willingly), appear to be familiar with her, and regard her highly...in unique ways. In Adventure, they worship her as something of a goddess, or even a mistress. In 02, that worship continues, in the form of them referring to her as the "chosen one," and then...wishing to bring her to their master as his bride to mate with.
To summarize, yes, I am saying the harmless, cuddly Numemon of Adventure were reborn as the eldritch monsters that stalked Hikari throughout her childhood in hopes she would have sex with their master.
Alternatively, as these creatures are able to take on the form of Digimon, they may have originally been under the guise of Numemon, giving a more plausible, less convoluted explanation as to why they didn't die regularly and become reborn as Digi-Eggs. But I sort of like the idea of Hikari's light having life/death attributes.
And that's my theory! You are free to take it as interesting food for thought, fodder for fic, or discard it entirely as a silly idea. I hope you enjoyed reading, regardless.