It is possible that that being is Hagormo's father. To understand why this might be the case we have to look at the myth on which Kaguya is based. Kaguya comes from the Japanese tale of the bamboo cutter where a couple find a you girl in a stalk of bamboo raise her, and eventually find that she was originally from the moon.
This Japanese story however is ultimate based on a Chinese story where Kaguya is replaced with the moon goddess Chang'e. Though the stories involving these two are slightly different they both involve the celestial woman ending up on the moon. In the story of Chang'e there was an archer named Houyi who saved the world when 10 suns rose in the sky and he shot down 9 of the suns, leaving only 9 and saving the world. Eventually he married Chang'e, but she drank too much of the elixir of life and floated to the moon.
The design of person shown seems to be very different than that of Hagoromo so I don't believe that it is supposed to be him, though the design could be meant as a metaphor. However, with Kaguya representing the moon and the "Houyi" character representing the sun I think there is a possibility that he could show up. We still haven't met Hamura and maybe when he shows up he will explain the the ultimate fate of their father, if he existed. Also the constant dichotomy of sun and moon, yin and yang makes sense and has been present throughout the manga and I wouldn't be surprised if Kishi took the dichotomy as far back as Kaguya.
We could see the progression of the sun and moon seals that goes back to the original users of Chakra Kaguya and "Houyi." Where Kaguya originally had the moon, Houyi the Sun. The passed their marks on to their children with Hagoromo getting the Sun and Hamura the moon. Something happened to Hamura so he gave his mark to Hagoromo, and Hagoromo in turn passed it on to the reincarnations of his children Indra and Ashura.
It seems that their isn't much time left, but it is possible that Kishi can explain this stuff.
