That Robert Browning's story "Chile Roland To The Dark Tower Came" ended with the main character blowing his horn before entering the Tower, and as Gan gives Roland the Horn of Eld to blow upon his next visit in Stephen Kings finale (since he lost it during the saga we read), it likely implies the next time Roland reaches the tower he will have completed the story, and will have better outcome for him when he faces Gan for (hopefully) the final time. With that notion, I don't think we've seen the end of Tim or Roland.
Bookmarks