So I noticed this about Shadesmar: all the things come in threes.
Sea of Regrets, Sea of Souls, Sea of Lost Lights
Nexus of Truth, Nexus of Transition, Nexus of Imagination
Expanse of the Densities, Expanse of the Broken Sky, Expanse of the Vapors
Three sets of three, even.
Considering we have three Shards on Roshar (and don't say we don't. I mean it. I will smack you), they could correspond to Shards. Or they could correspond to the three Realms--Physical, Cognitive, and Spiritual. Or both.
But we haven't done much theorizing about that Shadesmar image, so let's get to it right now!