How to fix this film...
I didn't hate this film, and actually admired how efficient and straight forward it was. I can understand that people found it predictable, but I dont think stories have to be unpredictable. If the acting and the way the story is told is done right, then predictability shouldn't be an issue.
I think what people take issue with more then anything, is when the characters behavior bother them. When they do stupid things just to move the narrative. When their actions don't feel honest or earned.
The biggest issues with this film are:
I. The inciting incident that moves all the action feels forced and ridiculous.
II. Some of the characters actions don't make sense.
So starting with issue number one. The whole dinner scene, and how the neighbors wife fell down the stairs came off as fake as hell, and was silly. I thought the film was setting something up and they didn't.
The easy fix for issue I. : is that neighbors baby had already died. They went to a check up, and the baby died from complications.
This then makes the dinner scene make sense. Why the neighbor is getting upset as the dinner goes on. Why she keeps talking about how she was picked because she could have a baby. And why she's upset that the Wife is pregnant, and doesn't seem to want it to the same level as she wanted a child.
Her falling down the stairs would then be planned. It's how they cope with the baby dying. They are delusional and don't want to take the blame for their baby already dying (genetics) - and use the dinner and stairs as a way to shift the blame and deal with the baby already being dead in her belly.
They could have easily added a 20 second scene in the ending montage, that shows her and the husband at the doctors telling them their baby has died and needs to be removed - then cutting to the dinner scene again where she's crying and being upset over the other wife not wanting her baby more.
Boom. This then makes the dinner scene make sense. It explains why she's getting upset. It explains why the fall down the stairs felt so forced and fake (rather then leaving it up to a cat, and the main wife not changing the lightbulb). It would make the scene more believable, and would set the stage early on for these delusional people shifting the blame - so that they feel owed the protagonists baby.
The Second major issue is the characters behavior / actions . It really never made much sense why the main protagonist is obsessed with the other lady. It's almost a dream like other worldly thing. They explain that she feels comfortable observing other people. But this feels like a contrivance to explain away her trusting these clearly unstable people. After the dinner scene, it made no sense why she was so hell bent on coming into contact with them again.
Had they showed her feeling more guilt, then maybe it would have made sense. But she instead comes off like a weirdo herself. And they don't really do much to explain this behavior. Maybe they could have positioned it where the neighbor was more of a mother figure, since she craved a relationship with her mom. I dunno (that side plot was also a waste).
Anyways, she's not the main issue I have with regards to character behavior. It's the husband. You see, the husband never liked them. He always felt off about them. Their behavior at the dinner and the stuff they said - was not normal (even before the accident). So it doesn't make sense why the Husband was so trusting.
Hell, I would have been sketch if my wife allowed our new son to be watched by these neighbors that I already felt weird about, and now have seen first hand how crazy they are. Especially since their issues are directly connected to them wanting a child, and how their child's death is related to them. So right off the bat, I don't believe why the husband wouldn't have rose a stink over this, or at least some concern.
Then when the Wife does start to say something is wrong, she has to prove it to him. The husband shouldn't have needed proof. Granted, the neighbors did set it up so the wife was "forgetting" things, and acting stressed. So to be fair, the Husband second guessing her is related to that. But there wasn't enough scenes to warrant him completely buying into his wife being the problem / and ignoring all that she's said about the nighbors.
The easy fix for issue II: Simple. There should have been scenes where the neighbors buttered up the protagonist. Where he got him tickets to an event he wanted to go to. Where they start to hang out. They already set up how he was rich and successful. They even feel the need to point this out in several scenes (without any pay off). Basically just a few scenes that show the neighbors making the husband feel at ease with them, and selling him on the idea that they were just normal people that were just traumatized. They could have even done something where the neighbor relates to the protagonists job (maybe he could have been his new boss. Or a client that brought him a lot of money for his company which then boosted his career). Like the dinner scene, the film almost seems to want to set something up, only to do nothing with it. They spend numerous scenes letting us know the neighbor is successful and business savvy, and with no pay off (this is also a pretty quick fix, that would have gone a long way to making the audience buy the husband siding with them and not the wife).
Then all the little things where the wife is forgetting stuff and being stressed - would play into the narrative that the neighbors are normal, and it's his wife that is having a break down. But the way the current film stands, it makes no sense why the husband is so willing and trusting - when the first half of the film he's never liked the neighbors.
This obviously won't fix every issue that some people had. But I feel they are very simple tweaks that would have alleviated two major problems with the film. And what is baffling is that, the film almost is aware that it could do these things. There seems to be set up, only for them to just go "nah" well just leave it alone. This might have worked if the film had twists, or was a subversion on the tropes/ genre. But it's not. It's actually a very straight forward story. Which i actually have no issue with. It's actually kind of refreshing. But it just misses the mark, by feeling slightly undercooked and missing out on the obvious.