Tell me what you think. Also, try to add up some of your own ideas and maybe in the end we'll create a perfect P3 plot 
P3 starts like 4-5 years after the movie. Naruto, Sasuke, Hinata and some other supporting characters from old gen are dead due to a plague of some lethal disease (killing off OP characters and the ones with Ootsutsuki connections is a must for P3 to even exist).
Sakura, Shizune and Tsunade in the end manage to somewhat control the contamination, but only to an extent.
It looks like a new cult has appeared - a group of Amatsu-Mikaboshi followers. Amatsu-Mikaboshi is an the god, creator of Jashin and other lesser gods. His goal is to take over the NV and expand even further (moon etc.)
To do this, he needs a powerful host and knowledge, which leads to a certain someone - Orochimaru. Orochimaru, who became the strongest living being after Sasuke's and Naruto's death, is supposed to become a vessel for Amatsu-Mikaboshi. That's why, with some sort of a spell, Orochimaru gets captured.
But to release Amatsu-Mikaboshi from his underworld prison, there are some requirements to be met. Aside from Oro's feats, the intel on Ootsutsuki, Kaguya, Uchiha tablet etc. It turns out that all the legendary tools come from the Ootsutsuki clan - Kusanagi, Yata, Totsuka etc. By capturing Orochimaru, the cult put their hands on the three tools mentioned (Totsuka and Yata by forcing Oro to use ET on Itachi). Still, there are 4 more tools hidden around the NV and only their combined power will allow Amatsu-Mikaboshi to manifest his physical form in Oro's body.
Ofc Boruto, Mitsuki, Sarada and the crew have to put an end to this plan. Mitsuki remembers how his father (Oro) lectured him many times about some secrets and legendary tools and stuff like that, but now it starts to make sense. Boruto and Sarada ofc want to avenge their dads. And ofc to save the world and Oro
So basically that's it
The story would revolve around trying to get to the find the tools before the cult does that, some plotlines around the cult trying to kill/kidnap Mitsuki or some other variations. Also, the plague thing would allow the story to make it believable, that older and stronger characters won't be stealing the spotlight. Ofc there could be a lot of minor storylines to make it more interesting. Not to mention that there's always a possibility of betrayal inside the cult, for example some member trying to get all the power for himself etc.
