The only thing I can tell you now is that one of the eggs Bill was destroying (it's been a while since I have read this part, hope I got it right) survived; Bill simply missed it. It eventually hatches and gives birth to a young It. After growing up a bit (which is fast, since time is different for the thing) it makes its debut in Derry on Halloween, falls in with a group of trick-or-treaters. It's dressed as a clown (or maybe something else, who knows). Eventually, it takes out all the trick-or-treaters it is with and goes to one final, isolated home and knocks on the door. A family is inside. It feeds.
That would be the prologue. The rest? I just don't know. I will say that there would be interlude parts, but instead of being the history of Derry, it is the history of It...as well as dispatches from the future of It. I'd also say the sequel should not be as long, or soap operatic (although having the children of the Losers fight the second It might be symmetrically satisfying).
Sequel? Nah...gimme a prequel. Something nasty that happened long ago, back when P-wise/SpiderMama was in his/her/IT's prime. What else was going on in that town during the years of the Kitchener Iron Works explosion, or when they gunned down the gangsters? The Black Spot? The time that logger went nuts and took an axe to some folks? Something like that, from way back when.
More of Mike Hanlon's Derry Interludes, basically. I would read some of that.
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.