And ADS-B style is probably the best and easiest option. It can be an easy add on to existing quads and probably done pretty cheaply. Places with airspace concerns can easily setup a base station to listen to their local area.
> Every single incident involving a drone has involved the concern of violation of someone's privacy or violation of an airspace, and even in those incidents there has been a lot of hysteria involved in the media response
They haven't yet (at least in a mass event, several have fallen on crowds etc) but violations of airspace, especially around airports, don't have to actually cause injury because the threat of an airliner engine eating one somewhere between their commit speed and their rudder authority speed or any time really during take off and landing requires airports be extremely cautious about drone sightings.