There shouldn't be a sequel. I'm not a party pooper, but that story has a beginning, a middle, and an end. Any attempt to make a sequel would invalidate all the accomplishments and sacrifices of the Losers (as well as others). It would also make a liar out of the Other which just won't do. What could be done, I think, is a prequel which takes place as an expansion of one of the dark interludes of Derry's history. An expanded story of the period of the destruction of the Kitchner Iron Works might work. I've often wondered if an earlier group of people that chose to stand against the monster died there (among the many). While they may not have stopped It, we clearly don't know what happened fully or why. Part of me tells me there are a lot of layers to that onion.