He opens the door at the top of the tower and finally becomes part of God Head. ENTER THE MIND OF GOD.
Finds that God is one of stephen king s tower junkies.
The actual coda is a dud made to fool whatever following red eyes has.
Or another good one is
Roland finds out the door was a former door that once led to the Source 'the matrix'. And that the Dark Tower world was once the main matrix simulated world before the Architect perfected our own world.
Long Live the Eraser