'The Light has struck a bargain with the enemy of all', - Nzoth presumably about Light and Death forces making the temporary alliance.
List of sightings:
-Uther Lightbringer, paladin, has relevancy in Shadowlands.
-Arthas, former paladin and Lich King, was thrown into the Maw.
-Bolvar, former paladin and Lich king, is a prominent character in Shadowlands.
-Anduin Wrynn, priest, was brought to the Shadowlands.
-Tyrande, warrior of Elune, has relevancy in Shadowlands.
-There are certain Light themes presented in Revendreth.
I dont think the presence of all these Light-themed characters in Shadowlands is pure coincidence.
My prediction is that Light will have 'unexpected' relevancy in second half of the expansion, smoothly tying it and next expansion (presumable, about Light (and Void)).