One of the most innovative, useful, and wildly popular features introduced into Unity 7 was the HUD. Unity 8 currently does not have it.
When Unity 8 was started, we had a way of accessing the HUD with a long-swipe from the bottom edge. However, the API to use it was late in being added to the SDK, and changed in incompatible ways after it was available, and as a result only a few apps (including one of my own) had taken advantage of it. Moreover user testing showed that the swipe-up activation was not easily discover-able by users.
When the toolbar/header was redesigned and the bottom-edge gesture given to app developers, the way of accessing the HUD was removed, and the API was removed before the first supported API freeze.
Before switching to Unity 8 on the desktop we will need to re-introduce the HUD into Unity, to avoid a feature regression that many have come to expect and appreciate. Moreover, we should re-introduce it on the phone so that apps can once again start taking advantage of it, so we can learn how to provide new use cases and user interactions for phone/tablet apps.
In order to match, as much as possible, the Unity 7 way of accessing the HUD, my recommendation would be to implement it like an Indicator, but always on the far left of the panel. Then it would be touch-accessible with a swipe down from the top, or switchable from within the drop-down of another indicator, placing it at the same screen position as it is on the Unity 7 desktop. Placing the window title or simple "Menu" in that location of the panel (or switching between them, depending on available space) would aid in discovery of the feature.
Apparently we need either a bigger hammer, or a bigger stake. ;-)