Invisibility is hard to do right, and complex (or at least non-exploit-happy) AI would be required. That's the biggest challenge... actually having some people go batshit and others staunch skeptics and still more who insist on finding someone to toss holy water about the place, without making everyone form-people... but perhaps the "You're a ghost, get people out" idea could be done in the same way... I've love to see something of that sort, perhaps?