I'll just post it here so I can say I told you so in 6 months time.
Its a bunch of ideas I had last christmas.
Basically its like an augmented reality seance or a summoning type idea whereby the more iphones/ipads being used or focused on the same area the greater the resolution/clarity/power of the summoning or summoned entity.
So you could go on a hunt or summon otherworldy entities- perhaps engage in a block war type thing. But the main thingis the more of you in a small group in the same 'game' the greater the power/clarity of your summoning- perhaps you need a number of people ghostbusters style to contain certain entities.
The idea would be that on one hand -the combined processing power of the units talking to each other provides the shared compute power to bring the thing into an impressive augmented reality life, but also to leverage the fact that often you've got a bunch of people or friends who've all got these type of devices in close proximity to each other. (often when they meet socially at night out or at someones place)
You'd use key real locations would be settings for various encounters, maybe localised .
So you'd have to go to a particular place at a particular time for the best chance of success- somewhere spoooky at night etc.....
You could maybe mix in some ghostly video transmissions being sent to people, or appearing/overlaid onto participants video chat.
Another riff on that mechanic could be revealing/decloaking a race of aliens/ships living amongst us-(parked in the sky above monuments etc)-and you 'need more people at once to overcome their cloaking signal' in a given location(a-la- 'they live').
This one about aliens is probably the most doable- you could have floating things in the sky and not worry about collisions so much, but maybe also have things crawling over buildings in the CBDs-(which are more likely to be mapped in 3d somewhere already).
I might do some roughs to illustrate the ideas of some of the monsters/entities or the aliens.