Ok I came up with some plot points for the Boruto series:
First off, in term of power i think the diamond curse mark/seal should give him a sage mode-like boost, being from an Otsutsuki and all, and then have him hear about Minato and grind to learn Flying Raijin cuz since Mitsuki already has sage mode we can diversify. As for his jougan, which seems spaced-time based, make it extremely limited in terms of use and very draining. Boruto should also be very potent with the chakra natures that he has.
Now in terms of plot, its very simple:
-Sideline Naruto and Sasuke, somehow.
For example since the third Otsutsuki is set to appear in the series have him come after Momoshiki is defeated, realize that he cant beat Naruto and Sasuke on his own and attempt to free Kaguya in order to aid him. That way you can have Naruto and Sasuke fight them in some dimension constantly for years to keep them far from the world.
That would give Naruto some new drive and send him on a mission:
-To protect Konoha, and the world in honor of Naruto and Sasuke
-To work hard to figure out how to get his eye to access the dimension where Naruto went
-To get strong enough to change the tide of the battle against the Otsutsukis
Additional Points:
-Toneri can come down to earth and help train Boruto
-In the absence of Naruto Kakashi can retake the Hokage position
-Boruto can travel the world trying to acquire more power, asking bijuus for some of their chakra, etc...
And there you go thats the backbone of the story. In the end, Boruto, age 20, finds Naruto and Sasuke helps them finally defeat Kaguya and apologizes for having been a brat all those years.