So ... Nightblood has to go. He's already the most Invested object in the Cosmere, capable of killing a (weakened) Vessel, far stronger than the mere Bands of Mourning per Brandon.
How can it be removed from the story?
Well, who's really good at disposing of surplus Investiture? How about Rysn's great friend and protector, the Ancient Guardian, Chiri-Chiri? He eats Investiture, and presumably will need more and more as he grows and develops--he, like Nightblood, has already absorbed enough Investiture to become sapient. (That's my explanation, anyway--we know that enough Investiture gathered in one place becomes sapient by WoB.) In fact, Larkins are rather like non-murderous Nightbloods, aren't they?
I predict [looks around to be sure no Vorin people are present] that Chiri-Chiri will end up draining a huge amount of Investiture from Nightblood. Might not kill him, but will most certainly weaken him. And like Nightblood, Chiri-Chiri never seems to be full.
Another prediction: Chiri-Chiri will form a romantic, or at least child-producing, relationship with Nale's larkin (who drained Lift back in Edgedancer).
Yet another: the Sleepless were worried that Rysn might become capable of actually exerting the Dawnshard power by becoming a Radiant. She agreed not to. But she does have a Connection to Chiri-Chiri. And if my first prediction is right, Chiri-Chiri will drain an enormous amount of Investiture from Nightblood and become, effectively, a demigod-dragon-flying-crab thing. Could the bonded pair of Rysn and Chiri-Chiri use the Dawnshard power?