I think I've thought of a way to have the Last Dragonborn appear in the next Elder Scrolls game. Imagine a Nord army, led by a Jarl of the next High King, basically, another chosen one of Kyne, destined to form the Second Empire of the Nords in the wake of the Mede Empire's collapse. At their side, you would find a cloaked figure, a warrior wearing armored robes and a golden tusked dragon priest mask. This would be Konahrik, translated to Warlord in the Dragon's language.
This character would be implied to be not the Dragonborn, but an aspect of the Dragonborn. Basically, you would not be sure if this was the Dragonborn, like Sheogorath in Skyrim. It's implied, but not confirmed. They would be a shadow of the Dragonborn, the real Dragonborn probably being in Sovngarde or the Hunting Grounds or the Evergloam, while this Dragonborn in the one part of the character that stayed in Apocrypha, he is the Champion of Hermaeus Mora, the part of the hero that stayed behind, the hunger for power. Like with Sheogorath, appearance doesn't matter. The Hero of Kvatch could have taken any form, but Sheogorath's personality took over and they took on his appearance, Konahrik could have basically taken the form simillar to Miraak, or perhaps Konahrik himself.
And why would you have this? Well, it's kind of epic, isn't it? Basically, the chosen one of TES VI meets the Last Dragonborn and the Hero of Kvatch, perhaps you could even have the two talk. And again, this wouldn't be the Dragonborn anymore, the exact fate of the hero would be unknown, but this would be the hunger of the Dragon Blood given form. And why would Konahrik help the new Nord High King? Well, blood is thicker that water. You could also excuse the whole being Konahrik thing with a Dragon Break or with the event that will probably happen in between Skyrim and Elder Scrolls VI.