Lurking in the developer options of the Android M dev preview is something called System UI Tuner. It's not a very descriptive name, but if you back out to the main system settings, you'll see it listed at the bottom. Open it up and you can customize the Quick Settings finally. It's something most Android OEMs have been doing for years.

The new editing interface lets you drag the existing icons around, remove things you don't want and add new tiles. Of course, there don't appear to be any apps that support adding tiles yet, so you're really only able to play around with the stock ones right now.

There's an option in the overflow menu to reset the quick settings, and you'll be using that often if you mess around with the UI Tuner. It seems like removing one of the default tiles breaks it even if you try to add it back. Resetting the list fixes it, but there's a reason this is tucked away in developer options. It could be great in the future, and maybe more parts of the UI will be included.