I was thinking of letting the story happen roughly 50 years after the events of The legend of korra and with a focus on The United Republic/Republic city. I think republic city will prove the best background for a story that revolves around inequality, social unrest and stuff like that due to the fact that it is a young, multicutural nation. It does not have the racial ties and tradition of the 4 nations. Plus the diversity of it's people can be cause tension. 

With technological advancements fueling the nations industry an new economic and political elite can rise that enriches themselfs at the expense of the others. As tensions due to economic inequality rises the government heavily regulate bending because it has been used during riots against police officers. Slowly the government has monopolized bending and the training of new benders in order to prevent the people from revolting. Ofcourse in such situation an underground countermovement will be formed and that can be where the players come in :)