The free flight concept has been around awhile, it's a lot like VFR isn't it? And the NLR has created a great video game. seem to recall them being big on microwave landing system.
Problems seem to be little matters like workload and coordinating everybody in a reasonable and efficient manner, which won't happen if the engineers and NASA (a huge public relations machine with some science attached) and R&D wonks would quit trying to be controllers and pilots. And controllers and pilots quit trying to be engineers (or God forbid R&D wonks).
Implicit assumption in most of the Free Flight stuff I've seen (mostly US, a little Euro) is that there can be something approaching perfect information on the position and intent of every aircraft that can be communicated to all the other aircraft. This is a tall order and not possible in today's technology and not practically possible in the forseeable future for every PA28 to 777 as has been pointed out in earlier posts.