Basing on what we were told by Kaguya during the final battle and by Momoshiki and Toneri in the two movies, the Otsutsuki Clan is an ancient clan of extra-terrestrials who have been travelling throughout dimensions for eons in order to find all the worlds that are cultivated with the God Tree; when they find one of them, they ate the fruits of the God Tree, so that they can make their chakra stronger and also prolong their lives. One of those worlds is exactly the Earth and they arrived on it about 1000 years ago; there, they started the history of shinobi creating the world of ninjas.
In particular, the first member of Otsutsuki Clan who arrived on the Earth was Kaguya, but, as we already know, at the beginning she didn’t want to destroy our planet; on the contrary, she wanted to protect it, so she ate the fruit of the God Tree in order to gain enough power to defend that world from any enemy who could threaten it.
Furthermore, Kaguya has been the final villain of Naruto Shippuden saga just because Kishimoto didn’t have enough time to include also Momoshiki, Kinshiki and Toneri in the conclusion; for this reason, he made them appear in the two movies “The Last” and “Boruto”, although nobody had still told us why Kaguya created the White Zetsu army and why Sasuke started to travel around the world.
In this photo you can see an image taken from a flashback of Kaguya which shows us her backstory; as you can see, the second shadow is Momoshiki’s one, the third shadow is Kinshiki’s one, but we don’t even know anything about the first shadow.
Well, surfing the net I found a statement of Kishimoto, who said that in the movie “Boruto” there should have been also a 3rd villain called “Urashima”, but there wasn’t enough time to make him fight. Now, following the logic of this theory, if this hypothetical “Urashima” would have come after Momoshiki’s defeat, this means that Urashima would have been even stronger than him; we also know that Momoshiki is very strong, in fact (after Kinshiki absorption) Naruto and Sasuke had to fight together in order to be able to defeat him, so Urashima should have probably been the stronger Otsutsuki clan member ever seen by us. And he’s also supposed to be the first shadow in the image we talked about before, since we don’t know if there’s some other Otsutsuki Clan member alive who could possibly substitute him in that photo.
In conclusion, I strongly believe that Kishimoto won’t make this character (“Urashima”) useless, so he will probably use it as a really powerful villain in the Boruto series, maybe also connecting him to Kawaki, since we saw that also Kawaki will be a major villain for Konoha.