When i first heard of how users of the death note dont go to heaven or hell i thought,where do they go?
I came up with a theory of the person who uses the death note become a shinigami itself.
Death Note volume 13:How to Read: The user doesnt go to heaven or hell but go to Mu,or nothingness.That is what the blank pages at the end of vol.12 symbolize.
Death Note Rewrite: the 2 hour special on Death Note behind the scenes giving more information than How to Read,this is told by Ryuk who explains to a curious shinigami what he did in the human world,similer to his log in How to Read.Spoiler Alert!In the end it turns out Light is the shinigami
Appearance: Skeltal body,blond hair,goggles with jewels in them,raggedy clothing,carries a sythe type stick. He brings Ryuk a dried up apple.
[Information from Wikipedia,Youtube,Death Note 13:How to Read]