Well it's certainly something that piques my interest. Avatar in general has many fun possibilities to entertain roleplay-wise, and, depending on how you go about it, an Avatar roleplay with a modern setting could be pretty interesting. Or terrible, if you do it wrong.
So pitch some ideas~
Well here is my basic plot for it, i have a little more detail to it, but I'll spare it for now:
1.) Spirits and Man live side by side.
Spirits can disguise themselves as normal people, making it so that their presence isn't as known. Only people with a natural affinity towards the spirits can see them for what they are. Think of someone like Jinora.
2.) As technology grew, people began to be "on par" with Benders.
Their powers, however, frightened people and a witch hunt for them began. People still know of them, but most benders try to stay incognito for their own safety--especially given that some benders who have publicly displayed their powers have strangely gone missing shortly after.
3.)Bending is essentially illegal, which is mostly a coup by the world's government to retain their power after they managed to bring down one of the most powerful bending organizations, The Red Lotus.
Knowing all too well that a small group of proficient benders have the capacity to bring the world to shambles, they worked together to bring down what has been labeled as an international terrorist organization, but they still don't like each other and their cease fire was ended shortly after they believed they had brought down the group.
4.)The Age of the Avatar is thought to be over.
One has not appeared in decades. The Red Lotus was not defeated, but knows that they must get the people on their side if they are to accomplish their goal and bring freedom to their fellow side. Their idea for this was to create a "fake" avatar that would serve as the rallying point for these people to turn against their governments.
5.) Of course, a real avatar happened to show up.
Since benders stay lowkey, s/he is actually attaining most of their knowledge through spirits who can sense that the world is close to waning into oblivion, especially being on the brink of an all-out war between the worlds greatest powers. His guidance eventually leads him to the realization of his purpose in this world, and sends him on a collision path with the Red Lotus who has begun to actively promote their "Avatar." The Red Lotus, however, can not appear as liars so they ultimately want to remove this Avatar from the picture, because it's not the Avatar they need, but the glory that comes with it.