Typically games use a messaging system similar to the sort of thing you'd expect implementing the Actor model. This lets you keep things decoupled whilst still leaving them the ability to communicate. It's pretty handy but also needs to be used with care if you're not going to invest a bunch of time in writing tools to support it as it makes debugging tricky.

Don't be too worried about mediators though. Games are a little bit different in that you typically want to act on a shared world. If you have systems that act globally on the world then you're just matching the data that exists. Even the physics system itself is basically a singleton that's mediating interaction.