Horror movies have kept pace with superhero franchises, stretching individual films into entire interconnected horror universes, with franchises like The Conjuring and Paranormal Activity leading the way. Insidious has established its own multi-branched franchise timeline, and according to Insidious: The Last Key producer Jason Blum, there's room for the Insidious universe to include another one his popular horror franchises: Sinister.
"I hope so, I want to. We almost did 'Insidious' and 'Sinister' and I still feel like we might do that, so I think so. I think we're going to cross our worlds at some point. I don't know how yet, but we're going to try."
In addition to dropping that bombshell, Blum goes on to talk about the future of the Insidious franchise, confirming that there will be a sixth installment (provided The Last Key earns at the box office), and possibly even a seventh, in order to extend the series past the six-film run of Paranormal Activity.
As for the crossover: It really wouldn't be hard to pull off, actually. The Sinister franchise revolves around a demonic monster (or deity) called "Bagul," who travels from some otherworldly dimension into our world via images of himself (such as paintings, photographs, or video) to consume the souls of human children. Meanwhile, the Insidious films have become centered on the otherworldly realm of "The Further" and its many different supernatural connections to our world across space and time.
The Further has offered many "red doors" through which various demonic entities have infiltrated our realm - it's pretty easy to see how that framework could fit the idea of Bagul traveling between The Further and our world, eventually catching the attention of demonologist team Specs, Tucker, and psychic medium Elise Rainier. In fact, given where the Insidious timeline left things off (with the ghost of Elise spotting a horrible new demonic threat in a young girl's house), it would be simple to flip that ending into the beginning of an Insidious / Sinister crossover.