I just finished binge-watching Season 1 and Season 2. Season 1 tried to stick to the book as closely as possible. There were a few changes in the characters, but overall, I thought it was pretty good. As for Season 2, They changed too much of the story, and I was a bit disappointed in that.
I see four possibilities:
1) They do Finders, Keepers
2) They do The Outsider - Jack Bender showrunner of Mr Mercedes has the rights to Outsider
3) They do something original.
4) They adapt the rest of End of Watch.
Check out my latest Stephen King Podcast Ep - #96 - to learn what my far out theory is as to how Option 4 - could be done.