My own concept for how the legacy should end
This is just a personal idea i came up with after playing Defiance again, but wouldnt it be amazing if Kain decided that the only way he could find the answers as to how Nosgoth can be restored....he travels back to the time of the war between the Ancient Vampires and the Hylden!
Think about, everything you see on those murals as Raziel you get to see first-hand as Kain. You get to meet many Ancient vampires, battle the Hylden, even have a face-off against the Hylden Lord (with a subtle reference to BO2, with Kain saying something like "i know you, i have defeated you before")
And the best thing (for me) would be that Kain would then have some monumental role in the actual construction of the Pillars, thus explaining why he is appointed as Scion of Balance when he is born many centuries later, and also explaining why he didn't die when his heart was torn out (because if he did die, the Pillars could never have been made and the Paradox would be tremendous). And it would then also explain why the Elder God wants Kain dead so badly, so that the Pillars would never have been made and the Vampires would not be cursed with immortality.
How's that for a concept?