Shows ranging from Game of Thrones to American Gods to even The 100 and The Vampire Diaries adapted and then expanded on source material; Castle Rock could have done the same.
The establishment of a multiverse in Season 1 meant that the sky could have been the limit, if the show would just take full advantage of the possibilities.
At the end of the day, I think Castle Rock could have become a big hit with a potentially long run if it had either embraced more of a serialized ongoing story based on one particular project or gone in more of an American Horror Story direction as a full-on anthology.