I would go with all flying machines being fitted with at least mode C.
It is all very well saying see and avoid which might be great in a pure VFR machine with good visibility and minimal instruments.
Take a twin stacked with instrumentation, poor visibility, engines obscuring the views and going a lot faster and he needs a back up to his eyes ie TCAS.
Gliders do pose a problem as they circle in numbers and being white in colour blend with the clouds and sky.
There is a problem or threat to aircraft like performance singles or twin aircraft flying in IMC conditions or in poor visibility and the pilots transitioning from instrument flying to VFR flying.
It is a case of "I am descending through cloud, I may be under RIS or RAS but can the controller know about a microlight or glider around the base of the clouds especially if it does not have a transponder? It is a game of Russian roulette whether you meet such an aircraft breaking cloud.
Or even worse flying IMC with no radar coverage which does happen a lot.
If its a case that we all fly VFR and in VMC out of controlled airspace at least we are all on a level playing field but its when the boundaries of VFR IFR VMC IMC overlap outside of controlled airspace that the threat of a collision increases especially with such a variety of aircraft types, speed and pilot capability.
Unlike in a car it is also hard to determine where another aircraft is going. a small speck can appear to be heading in another direction but can quickly turn into a screen full of aircraft or can be obscured by a high wing an engine or a turn.
We probably can all remember an occasion that we got too close and there for the grace of god go I to aircraft that do collide.
Maybe the solution has to be a mix of techology, a stronger boundary between VMC flying and IMC flying, more visible aircraft colouring, better communication and more advancement in the fitting of ballistic chute systems so that you do stand a chance if it all goes pear shaped.
Pace