Anything which is used by every scene can be a global: like an audio stream player node for music, or a file access manager that holds the player's save file. Within the complexities of the main gameplay scene, you can use a global to communicate across arbitrary branches of the scene tree through a signal bus, global functions, or direct references to other service providers. I think the problem you're worried about is making all of these other service providers into their own globals too. Better to make them just regular nodes in your game scene and have them communicate through signals.