Insidious is in the same universe as Silent Hill 4 & possibly other games of that series.
The further is essentially the Otherworld, a world of someone's nightmarish delusions come to life. Much like Walter Sullivan, the supernatural villains build their own lairs tailored to them. On this note, The Man Who Can't Breathe was doing more than just keeping souls as pets, they were also strengthening their own Otherworld. Much like South Ashfield Heights, the apartment complex in Chapter 3 was being corrupted by the Otherworld and Quinn was unfortunately trying to communicate with her decease mother where a soul collector happened to be listening in.- It's interesting to note in Chapter 1 that Dalton's room coincidentally has a ceiling fan much like Henry Townsend does in his apartment and both are prevented from waking up due to being imprisoned in an Otherworld in some capacity. It's also notable that Henry starts having hauntings in his apartment, and debatable if it's because of his empty body is attracting Walter's victims just like with Dalton's attracting Parker Crane's.
- Crossover or not, one of the staff from the Insidious films might direct a film adaptation of Silent Hill 4 and bring the best of both universes into the same film for a unique experience. Someone intimately familiar with working on Insidious could be well suited to doing a bizarre adventure through the astral plane justice and maybe even have Lin Shaye play a medium trying to communicate with Henry and warn him about where he's trapped while is body is in a coma, and help Henry in the final battle with Walter Sullivan.
The Man with the Fire in His Face isn't a demon per se, but a deceased ghost who was depraved enough for Elise to interpret as one.
This deceased person was very good at masquerade, and got great glee out of being seen as some powerful demon. This didn't make them any less dangerous however, as they nearly got hold of Dalton's body for a reason. The metal claw sharpening we see was from their love of A Nightmare on Elm Street and their stalking of people's dreams/astral travels was derived from Freddy Kruger's own exploits; unlike Freddy, they're interested in possessing bodies over killing people in dreams. With The Further operating outside of time and space, they got the chance to learn about their share of horror stories from all across time.- The hand claws were one of this killer's favored murder weapons in life, from watching plenty of Freddy Kruger, but he also enjoyed Darth Maul and styled himself after that.
- Unlike most people who die, they welcomed death as a gateway to immortality, and hoped to use The Further as a base of operations to hijack bodies for their own use and continue their insidious deeds in the physical world.
The girl who killed her family is Michelle Crane.
The daughter of the smiling, 50s-esque family who shoots them all is a young Michelle. She either got a light jail sentence for the massacre due to her age or somehow convinced police she was innocent, and so was free by the time she gave birth to Parker. Slaughtering her whole family as a teenager seems in line with how deranged of a person she is. Carl comments that a lot of terrible things have happened in the Crane house, this just being another of them alongside all the child abuse and serial murder.
Dalton will have to face and defeat The Red-Faced Demon.
However, in a cruel twist, Dalton will absorb his essence into his being and return part-demon. However, in a case of Blessed with Suck, Dalton will be better able to fight off demons menacing others. However, Dalton must proceed with caution using this new power, preventing it from being a Story-Breaker Power.Possession of an unoccupied physical body can be mitigated by astral projecting while in a vehicle, or in a protective location.
It's an idea that probably hasn't be considered in the Astral Projection film genre, but it could be interesting to see how beings in The Further try to solve the puzzle of catching a moving vehicle. Someone might get the idea to construct a maze, like an ancient pyramid might have, to confuse intruders.