I had an idea for a Omori AU, or alternate universe, where everything is the same, but headspace is a shared dream that Sunny and the rest of his friends have been having for years. Basically, outside of Mari, all the dream versions of characters are actually just them dreaming too. Headspace is also a more overtly supernatural and eldritch place that has existed for a very long time, and has been accessed by many people centuries before Sunny was even born.
Headspace is a realm or separate reality shaped by the minds of all the dreamers who have ever visited it, so there’s more inhabitants, places, and creatures/nightmares then in the game itself because of there being multiple dreamers. There would be many more forgotten places like the lost library too, places shaped by long dead dreamers where the echos of their thoughts, memories, and feelings can still be found.
Something would also be a parasitic entity that feeds off the misery, suffering, fear, and guilt of dreamers, morphing itself to become whatever they fear/regret the most and tormenting them until it completely drains them of their emotions, and leaving an empty husk in both headspace and the real world behind. The only way to overcome something is to come to terms with your fears/guilt/regrets and accept them, because something only has power over those trying to suppress, forget, and run from those things.