There are different asspects to analyze a V2G system,
From
EV point of view
Charger technology for bidirectional power flow (bidirectional switching cells converters)
reducing the losses during charging and discharging
mitigating the harmonics imposed by charging/discharging process
Battery degredation imapacts of V2G and the ways, technologies and stretegis to limit the negative impacts.
Protection for V2G system (Anti islanding, relay calibration, etc.)
Grid Point of View
Charging/Discharging Coordination of electric vehicle to provide services to the grid.
Frequency regulation
Voltage profile improvement and loss reduction
peak shaving/valley filling
load leveling
reactive power compensation (P, Q control on EV charger side)
renewabble energy intermittency mitigation
...
EV owners point of view
willingness to participate on V2G
economic benefits
economic feasibility justification
...
These are the example of different point of view that could be interesting to analyse. all the aboves are new, challenging and under studying by different industrial and academic sectors in the world. V2G by now is a technology that will be impelemented in next decades. there is a long way to reallity and research is still on going ...
I hope it would be helpful. I suggest also to follow the works of Paul, he has some interesting publication about V2G.