Was it a few months ago that Dutch pilots reported that when things got really busy on a good soaring day, ATC asked everyone please switch off your transponders, guys, we can't cope with that density of blips!
Well, uhm, yes, that's the gist of it. (It wasn't actually about glider pilots, but powered pilots, all near/under the approaches to Schiphol Airport. So now transponders are mandatory but you're not allowed to switch them on in certain places... Not a proud moment in the history of Dutch aviation. I hear that the situation should improve soon.)
But to be honest, I think that's the sort of information that the survey would need to include as well. If everyone and everything that flies is going to carry some sort of device that transmits its position (whether that's FLARM, ADS-B, mode-S or something else) there is a significant risk of information overload. Both for ATC and for pilots who rely on TCAS and similar.
You are going to need some clever technology and some clever operating procedures to make sure only the relevant information is shown. For ATC this means a better filtering capability so that irrelevant blips can be filtered out, and better heuristics as to the on-screen placement of the data blocks that belong to a blip. (The main problem in NL was that the data blocks were always overlapping, so information could not be read on the screen.) You may actually need to revise separation minima - the current minima are based on current technological capabilities. Once you know better where everybody is and what they're doing you can maybe bring the minima down.
But you also need clever operating procedures. For instance only activating the transponder once you are released from the winch. 'cause if you are already transmitting during the winch launch, TCAS onboard an airliner is going to extrapolate your zoom climb and issue an advisory even when the airliner is at 5000 feet and your winch launch is going to run out of steam at 1500'.
And of course there's the practical issue of battery power in a non-powered aircraft, getting a proper ground plane for a transponder antenna in a composite aircraft, finding space for all this stuff in a limited panel and so forth. Oh, and getting pilots trained in the proper use of the equipment: Last year when I did my weeks gliding course, the transponder was turned to "alt" virtually as soon as the planes were rolled out of the hangar in the morning, and was not touched at all throughout the day. Even when the planes were rolled aside after landing and not being used for a while, the transponders were happily responding to interrogations. Not an optimum use of battery capacity, and not a good operating procedure either.