I'm sure this wiki had user blog pages a while ago but I cannot find them, so I'm posting this here.
As we all know every Otsutsuki so far had their name and abilities based off a popular mythological figure from japanese folklore.
Now from the destroyed monument we can assume that there should be at least one more unknown Otsutsuki, who may appear later in the series.
Narratively, if he were to become the final antagonist after Code, it would be fitting that he'd be the King of Otsutsuki and the strongest member of the clan. He also may or may not have already achieved the many-eyed "God form" Isshiki showed to Code.
Here's my speculation on which mythological figure the Otsutsuki King might end up being based on:
Abe no Seimei, a famous Heian-period magician whose life has become the basis for countless myths and interpretations in modern-day popular culture.
Here's why he would fit right into the Naruto/Boruto universe:
The historical Seimei was a master Onmyōji (Onmyōdō practitioner) which is basically a form of magic centered around Yin-Yag stuff, divination and alchemy. Naruto already explicitly referenced this with Onmyōton.
Divination generally deals with obtaining knowledge through occult means, e.g. predicting the future, reading one's fortune (Karma), seeing that which cannot be seen by ordinary means. This ties into the concept of the Jōgan and Ocular Jutsu in general, the Seimei figure appearing in Boruto could be the progenitor of all this.
A central goal of alchemy is obtaining immortality through self-cultivation. This is exactly what the Otsutsuki strive for as well. In alchemy, the key to this is often the legendary Philosopher's Stone. Naruto's equivalent to Philosopher's Stones would be the Chakra fruits.
Lastly, check out this filler dude:
He appeared in the Part 1 anime's last few filler episodes and so ironically was the final pre-timeskip antagonist.
His name was obviously an allusion to the historical Abe no Seimei as well, only his clan was one of artisans creating magical Ninja weapons, not alchemists.
Funnily enough, he was already dead and had to be resurrected by the leader of his group of filler underlings becoming his "vessel" and sacrificing his own life for Seimei to be reborn.
He looks like an Otsutsuki and had floating balls similar to Truth-Seeker Orbs
Now I'm not saying they'll outright bring this guy back for the Boruto finale, but I could see them going for another Abe no Seimei reference and maybe a similar design for the hypothetical Otsutsuki King.
Who knows, maybe Kishimoto or Ikemoto even looked at those old filler episodes for inspiration when designing some of the existing Otsutsuki members.
