Yeah, do tell your idea. Would be interesting to hear. Been on a LoK kick again recently.
Well, the idea for Dark Prophecy was to continue the dual protagonists separated by time mechanic from Defiance, but this time it would be the Younger Kain (post BO2) and the Elder Kain (post Defiance).
We also know that part of the game would involve Kain finally visiting the Demon Realm (we see it briefly at the end of Defiance and it was mentioned pretty often in SR2 and BO2) and kicking the shit out of the Hylden in their slowly-crumbling Glyph Cities.
Also, the very title of the game suggests the return of the Seer - that weird uncorrupted Hylden woman from BO2.
In my opinion, going to the Demon Realm to destroy the Hylden in person makes more sense for the Younger Kain. He had just recovered his Soul Reaver from the Sarafan Lord/Hash-ak-gik, closed the portal to their realm, but heard his adversary vow to never cease their attempts at invading Nosgoth. In order to secure his future empire, the Younger Kain needs to neutralize the threat at the source, and destroy the Hylden Glyph Cities and everyone inside them. Thus, he finds a way to travel to the Demon Realm, and kick the shit out of them.
The Elder Kain, having been purified by Raziel's Spirit Blade, will go on the exact opposite sort of adventure. He will part with the Soul Reaver, placing it wherever it is supposed to be in the timestream, and will be confronted by the Seer. Kain's goal is now to destroy the Wheel of Fate, believing it will free all souls in Nosgoth from the Elder God's tyranny. The Seer delivers her Dark Prophecy - that if Kain succeeds in doing this, he will end up irrevocably destroying Nosgoth and irreparably damaging the very fabric of reality.
She then sends him away to the Celestial Realm (we saw it briefly all the way back in Blood Omen 1 when visiting Avernus Cathedral - the Dimension Guardian took Kain on a journey across many dimensions, one of which looked heavenly), and there he has to go through a number of Elemental Forge-like areas (Temples?), that will unlock knowledge and virtues that Kain had never had a chance to have, being afflicted by Nuprator's Curse since birth.
Each Temple will take Kain on a mystical journey, where he notices himself being watched by a distant and silent observer... who could that possibly be?!?! With each Temple cleared and an insight gained, the mysterious observer becomes more and more visible... until Kain realizes exactly who it is.
Eventually, the two stories have to converge somehow... I imagine the Elder Kain will eventually return to the future desolate Nosgoth that he left behind in SR1, but not by the means of a time portal, but rather by going into stasis in a secret area, and emerging from it as a blue-skinned, winged, True Vampire - the pinnacle of his evolution.
From there, he will make his way to the Silenced Cathedral and activate it, destroying the horrible monsters that his progeny had devolved into. In the process, he will also be cured of his curse of undeath, becoming the first living Vampire in aeons.
Finally, he makes his way to his throne - the ruined Pillars of Nosgoth, now understanding that the Wheel of Fate is not the enemy, but rather the Elder One - an ancient parasite that corrupted it long ago. He also understands that his role is not to restore the Pillars, but to undo the binding and remove the need for the Pillars' very existence. Before he has a chance to do that, the Elder God's tentacles erupt from the underground chamber where he lurks, and the final battle ensues.
P. S. Probably, Kain and the Seer end up forming a couple, creating hybrid Vampire-Hylden children that continue the ancient bloodlines, in a newly restored Nosgoth.