So what level will Tenseigan Chakra Mode be if we so implement it? I haven't seen the movie myself, so maybe I missed something, but as far as I know, Naruto is just as powerful as he was in the last chapter. With that being said, the wiki states that Tenseigan Chakra Mode is on par with Naruto's Sage enhanced Kurama Cloak, combined with that it gives the user attraction and repulsion abilities, strong enough to pull the moon itself, techniques that have celestial body destroying capabilities, and along with that Truth Seeking Balls as icing on the cake.
I'll be optimistic and say we can work with this.
If Tenseigan is allowed, then I say the user needs to have the Sage reset (and Byakugan of course), but completely relinquishes the Natural Energy aspect the Sages have, and we treat it as a power-up on par with it in terms of jutsu enhancement, speed, and durability. We treat the chakra costs of using Chakra Mode as one who is using Kanseitai Susanoo and give the Truth Seeking Balls the same level of destructive capabilities (except the ninjutsu negation of course). Silver Wheel Reincarnation Explosion though should just be as strong as your standard Planetary Rasengan and they have access to Shinra Tensei and Bansho Tenin.
As for obtaining it? I've got an idea, we make Hamura's Altar a claimed item much like Edo Tensei, Hiraishin, etc. The person who wants Tenseigan sacrifices their Byakugan eyes to the Altar and it transforms into the Tenseigan, in return you get all the above cool abilities but sacrifice Jyuuken and 360 degree Kaleidoscope vision. This limits the amount of people who have it, while taking a bit of Sage Mode and Rinnegan without the the specific advantages of either, some semblance of canon material, and evens it out as a Hyuuga+Otsutsuki version of Senju+Uchiha we have going on here. At the same time it leaves some advantages a regular Byakugan player would have. Sound fair? (and of course no combing Tensei with any other Doujutsu)