This is a feature I miss from Power Auras: being able to make one display conditional on another display being active or inactive. There are a number of uses for this:
* Letting one display override another. For fulmination tracking, for instance, I would like to show my lightning shield stack count most of the time, but override it with an icon when one of my other displays determines that I should cast earth shock. Since the icon doesn't really occlude the stack count, group priorities don't handle this (unless I'm missing some kind of occlusion feature).
* Boolean triggers (ticket #203) without requiring a boolean mini-language. By creating displays with no visual and making other displays conditional on them, you can make displays that appear when X and (Y or Z) are true, or things like that. Non-visible displays can be reused by multiple visible displays, providing a rudimentary form of abstraction.