Will there be a sequel to the Imager trilogy? A sort of tying up of loose strings? (I do realize stories never end, just morph)

It’s extremely unlikely that there will be any more Imager Portfolio books. Each of the four plot strings in the twelve books is as resolved as they can be. Each of those primary protagonists has consolidated his power base to the point that any serious opposition is unlikely in his lifetime. While it’s technically possible that I could create yet another plot string, I personally feel that would weaken the overall strength of the Imager Portfolio.