Very unlikely to be mandated for GA here.
Even Mode S is not mandatory for VFR for many/most areas.
You can already achieve much the same thing by enabling what is in effect partially
Enhanced Mode S, by allowing your transponder to radiate your GPS position. AIUI, ADS-B gives you just that, except it radiates your position continuously even if you are not interrogated by SSR.
Yet, any form of Enhanced Mode S is explicitly banned by EASA, on aircraft on which it is not mandatory
I recall going to a Eurocontrol presentation in 2008 where they said they want to dismantle primary radars to save costs and replace them with ADS-B. These people live in their own world...