If you know of the One piece cover story CP9's independent report then you know that during the time skip they were on the run from the world government. And if you're caught up with the series you also know that CP9 or at least Lucci and maybe Kaku are back working for the world government as members of CP-Aigis0. This will be a fanfic attempting to fill in the blanks. As we know the world government is made up of 170 nation's but what if some of those nations were a few we know and love from many other shows we know and love.
For example you can have them to the part of the world where avatar take's place and it can be during Aangs time and they help him inadvertently or directly put a stop to the Fire nation or they might decide to help the fire nations and go after team avatar. Or make it take place in Korra's time and they lend a hand against one of Korra's many enemies.
You can also have it take place in the Akame ga kill side of the world and have them as members of night raid or against night raid or make it neither and have them be spectators occasionally pitching in doing nothing unless they feel like it but willing to fight enemies who endanger the empire's citizens as a whole so they'd go after Zanko or Wild hunt.
Another idea you can have them go to the shinobi part of the world and have them help against the akatsuki.
Or just a thought create a series of one shots showing the teams reaction to different cultures and customs of multiple societies created by multiple authors as they are on the run from the world government.