I didn't think about it until you said so, but yeah, Kellhus could be alone with Seswatha's heart...would they converse??? That would be cool. The heart might know things that the individual presences didn't know, like if the heart gave each Mandati a customized version, refraining from pouring all the secrets into just one.
It would be cool if we find out that Mandati souls neither go back to heaven or hell when they die--they join the heart and add their strength to it! Seswatha sowed the seed for a megaweapon that would be ready just in time for Mog's return! Seswatha's Heart is the new Heron Spear!!!