There are a number of different ideas they are floating at the moment, but what is definitely true is that the current regime of subsidy on purchase, zero road tax and standard electricity prices will not remain in the coming years.
All of these things will be swept away "when the EV just becomes 'the car'" (as the government propaganda is telling us).
I'd guess we'll see these measures in place as soon as EV's reach 20% of the vehicles on the road - not too far away I'd guess.
Some of the different options being considered are outlined here:
The next thing after tailpipe emissions btw is particle emissions from tyres and brakes. Expect to see low pollution zones where electric vehicles are given a pollution rating and you pay a pollution charge to drive a 4.5 tonne electric peasant crusher vs a Renault Zoe. Probably also a significant scrappage charge on EOL battery packs.
One thing is for sure though - all those people that imagine they are escaping the endless tax war on motorists by buying an EV are wrong.