I doubt we will get a definitive answer even when an encyclopedia comes out.  It will probably be one of those mysteries that RJ thought was totally obvious, but the rest of us didn't pick up.  Or perhaps that was his point was to leave it up to interpretation.  Since Nakomi knew what Rand needed to do, I think it makes her a little more special than some wandering spirit.  For me, a Creator avatar or an effect of the Pattern to create a balance works just fine.

I don't think there is enough info in the few passages about Nakomi to completely figure out her role.  However, the creator avatar / mother earth type fits best for me.  She provided prodding for Aviendha to go into Rhuidean again.  She seems to be omnipotent and hints at what is going to happen since he is carrying Moridin.  "He could...see, just faintly.  A figure kneeling down beside him.  "Yes," a woman whispered.  He did not recognize the voice.  "Yes, that's good.  That is what you need to do."  He blinked, his vision fuzzy.  Was that Aiel clothing?  An old woman, with gray hair?"  Somehow she seemed to know that he was going to do something with the body he was carrying, or the creator, or she did it for him.  But that's all just my theory anyways.