Xemnas is an anagram for Ansem plus the X
But Xemnas can also be an anagram for names plus X.
The X of the organization names is "very probably" not an X, but an greeck X (chi) as seen on Vanitas X-blade inside the game it's sound goes a little closer to the word "key"
So Xemnas would thus became the key of names, this was seen when he gave Roxas his name from Sora's
That could also imply that at least at some point, Roxas was the key for Sora, as in Sora would not be able to wake up without Roxas (KH2)
Then Xehanort would be or keep the key to No Heart
Just some theories.