Kicking off with this.
Despite the beard, the resemblance to Nau-Cayuti, his ancient cousin, was unmistakable.
Enough to make Celmomas think he is seeing his son in his vision as he dies, perhaps? Perhaps a hint that Kellhus has authored the prophecy that he apparently fulfills by traveling to the outside where we know that time passes 'differently'.
In TDTCB, when Akka meets Kellhus he thinks he knows him and recalls the resemblance to Celmomas. This certainly seems to erode the theory that NC is actually the son of Seswatha, at the very least.
Then we have the theories relating to NC as somehow being Mog, and the theories that Kellhus will become Mog. Relevant and related, perhaps?