As a consultant, we proposed a hidden mickey photo safari/hunt concept once and I was surprised at how many there were and how widespread they had become.
I'm most interested in making some kind of guest accessible themed "underground" that only the players could find, or have access to. If they themed a tunnel from one land to another, there could be magic flowing through pipes, then this cavern or transition tunnel could be full of magical effects, etc. (Through a window we see all the "Presidents" in T Shirts, having coffee and Sun Chips in a break area,..ok maybe not.)
Imagine creating some kind of story that Walt had collected some precious historic items and had them in a vault below the HOP and we discover it with the "Disney GPS" on our phones. Or we end with a code that opens up Walt's secret apartment at WDW full of clues and collections? It just seems fun to me and something I'd go out of my way to do.