someone we have not met yet.

I would like to see it be Kevin. It's like this: he is a ghost, he is caught in the veil so he can't move on.

We have two canon things we know here:

1) Ghosts have a tendency to go crazy.
2) Prophets have a pretty tight hold on prophecy and an archangel to back them up.
3) Kevin knows what is on the tablets.

So a crazy prophet, who can't be replaced maybe, because maybe prophets aren't supposed to become ghosts? Who has potential ghost powers and a really strong support network.

Results:

The propheting is no longer being done. Does this mean that Dean and Sam, no longer held to the whim of the writing of the Winchester bible, for once in their existence actually DO have free will? What will they do with that freedom?

A lack of prophesy allows [some mcguffin] to happen that needs to be stopped.

Or worse a crazy prophet writes crazy things that then actually occur.

Or the archangel, manipulated by Kevin becomes the big bad of the season.

Or Kevin uses the power of the tablets to cause a breakdown of the walls between each reality.