I am generally in favor of a post Korra timeline. Either after her death, which means the new Avatar hasn't been found or has only recently been recognized and thus still in training, or during her life, but she's on an extended tour of the spirit world.
I think there are far too many opportunities post-Korra - post-Aang, even - that are enticing and can draw RP. Pre-Aang, female waterbenders are limited to learning healing, and the Fire Nation is default the enemy, and there are no air nomads. Technology is less a thing, robbing non-benders of a potential hook.
Also, post-Korra - how did we forget pro-bending?! If folks wanted to form a Bending team and do RP around that, it would be incredibly fun. Now add adventures in the now easily accessible spirit world...