I like where WB is going with his original question. And why not, fundamentally it shouldn't be that hard...

1) Have all ADS-B, Mode-C, Mode-S, and Non-xpdr radar targets get stored in a common database.
2) Through an app on a cell phone, have it downlink your current position, velocity, altitude, etc to the same database.
3) Through the same app, have it upload all targets from this same database that is within your vicinity and graphically display them.

The only hardware needed here is a cell phone. The only connection you need is a cell tower. The hard part is getting access to all the ATC surveillance data in one database. And the main limitation on the user would be connectivity to a cell tower (which I have found to be quite reliable in the air). If you look at this from a "basic situational awareness point of view" (sans collision avoidance), it would be very powerful and everyone would use it.