After defeat by the hand of Naruto in the Last and learning the truth about Hamura's will, Toneri decided to stay on the moon and atone for his sins.
However during Boruto timeline we saw him reappear during episode 8.
He noticed a comet nearing, realising that an important moment was coming and that Boruto Uzumaki is the key. Later, he approached Boruto in his dream, warning him about an impending danger. He also noted that Boruto's right eye is the key to stopping a great darkness.
Being that Toneri is (as of now) the only living person that had complete mastery over both Byakugan and Tenseigan (It would seem Boruto's eye is something between those two) and knows that Boruto's eye is a key to the incoming darkness (suggests he knows what the dojutsu actually is) do you think there is a chance of meeting between the two and maybe Toneri training Boruto in using his eye?