There is no denying that at the end of this series, or even before that, many people will be disappointed. It's all about expectations. I feel that ultimately a lot of the pulp elements of the story are just there to create artificial excitement - to generate cliffhangers, make it seem like a whodunit, have hooks that people can casually latch onto and talk about, but the real meat of the story which the author wants to tell is a story about a guy regretting how he didn't maximize his childhood, and getting the change to go back and be a better person. In doing so, maybe in the future he will also be a better person. Everything else is really just fluff.
That is honestly what I want out of the show. The actual mystery stuff isn't that interesting. The killer is either the really obvious choices like the teacher, or some complete nobody which who cares at that point. The real interesting stuff for me has been the Satoru, Sachiko, and Kayo dynamics.
The best thing the show can do at this point is keep the mystery stuff simple. I don't want to see overly convoluted time travel shit or any big dumb twist for the killer.