First, I'd get Studio Mir back, and get far away from Nick.

Second, it would definitely be set not long after the events of the finale. Team Avatar is only just a few years older and wiser. Korra and Asami's relationship isn't overly romanticized, they have hiccups and speed bumps like any other. The democratically elected government of the EK is still in it's infancy when a new enemy attacks. Hoards of spirits flood into the physical world, destroying everything in their path. No one knows why. Season one deals with Korra battling her way to the new spirit portal in an attempt to close it, and ends with her death.

Season two starts with a new avatar being born in the EK. The surviving members of the White Lotus, along with the parents of the new avatar all sacrifice their lives to deliver the baby to Team Avatar, who have set up base on a remote island. We learn that thanks to medical advances made by Future Industries and Varrick Global Industries, they were able to revive Korra from clinical death. Raava's spirit jumped to a new avatar before Korra was resuscitated, leaving Korra only as a waterbender. With the new avatar only a baby, and the entire world plunged into unprecedented chaos by some unknown force, Team Avatar needs to survive on the run, and keep the new Avatar alive, training him as he grows.

There will be stand alone episodes, but the total story arc will take place over an extended period of time during which we will actually see the new Avatar aging slightly. The series will be focused around this one incredible enemy, Korra's continuing personal development in the face of losing a major part of her identity, and Team Avatar's desperate struggle to survive while training the new avatar to restore balance to the world.

That's about as far as I've got. I'm still working on the details of the new enemy, they may not be spirits at all, but dark beings from another unknown realm which was breached with the creation of the new portal.