TCAS isn't going to help with microlights and gliders and non-radio (and hence negative xponder) a/c generally. If anything, the recent Herts accident involved two styles of aviation that are blessed with great vis and don't tend to fly head down. I think there is no 'solution' so long as flying is to remain open to a diverse range of classes and types.
Will Mode S be required on vintage, microlights and gliders? If so then that's a start of an electronic solution, albeit not a complete one as I don't believe that Mode S "enhanced" mode will be required in GA. Enhanced mode would be required for traffic detection..I think...can somebody expand/correct?
Also, I am suprised at how many a/c don't have or don't use strobes in daylight. Maybe a single small, slow strobing xenon daylight only light, top and bottom would help.