Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.
The Grabber’s ghost is now trapped in the basement and will play a part in any sequels
Say, if a family were to move into his old house.- In turn, the ghost boys may appear too and try (perhaps in vain) to warn those who enter the house about the Grabber's ghost.
The Grabber himself was once the "naughty boy" to a Predecessor Villain
Perhaps someone who lived in the house before he did.- Screenwriter C. Robert Cargill discusses this in a way that essentially confirms it
— the implication is that the Grabber is in some ways reenacting his own experience of past abuse (physical, sexual, or both) at the hands of likely a family member. (Max may well have received similar abuse but the consequences for him are much more ordinary — substance abuse and a hard time keeping a job, while otherwise being a perfectly decent guy.) The use of the belt as a weapon also echoes the beating Gwen receives from her alcoholic father.
Finney and Gwen's mother's visions eventually led to her demise
Which is why their father holds such disdain for Gwen's abilities.- Confirmed. They led to her taking her own life after they told her to, although it's unclear if these were actual visions or just mental illness.
Gwen's prayers weren't heard by Jesus or the like, but by the Black Phone itself
More likely, Jesus did hear her prayers and used the Black Phone and her dreams to help. In the Book of Habakkuk, God relates His intention to raise up Babylon - a “ruthless” and “dreaded” nation - to achieve His purpose. God is not the author of sin, but He can use sinful men to attain an objective.- ...But by that logic that would mean the ghosts themselves are sinful; granted two of them were confirmed shitkickers in life but as one of the two was at least a good friend to Finney it seems overly harsh, as well as only intervening after five boys were horribly killed.
If there is a sequel, it'll be set in the 1980s
It'd make sense, since the film was the late 70s. It would also account for the time it would take to make a sequel. The Grabber's first name is John.
Shortly after kidnapping Finney, whilst telling him he won't hurt him any further, the Grabber calls him "Johnny" ("You don't have to be scared, because nothing bad is going to happen here. On that I give my word, Johnny."). Going by the above (quasi-confirmed) theory that the Grabber's kidnappings are reenactments of abuse he himself suffered as a child, it's possible he sees each boy he kidnaps as himself and refers to them accordingly. It would also tie into a possible historical basis, the real-life serial killer John Wayne Gacy, who abducted and murdered numerous teenage boys in the 1970s.