Well, I'm stealing a plot notion from another fanfic here, but how about Hidden Waterfall and their Hero Water. I could see Gai wanting to get a look at it, to see if he could figure out how to moderate the damage done to a body by opening the Celestial Gates. Hero Water, after all, has a similar effect but has harsher side-effects. So observing it's effects could be useful. (The fic that suggests this is Set in Stone ( www.fanfiction.net/s/1730446/1/ ))
So the team might be on a mission to Hidden Waterfall and Gai has an excess of enthusiasm and tries to get hold of some Hero Water. Naturally, this goes wrong. And then Aoi turns up. If Aoi got away, he could also be the replacement Gatou hires once Zabusa changes sides during the Wave Country mission.
I like that team picture, btw.
D for Drakensis
You're only young once, but immaturity is forever.
So the team might be on a mission to Hidden Waterfall and Gai has an excess of enthusiasm and tries to get hold of some Hero Water. Naturally, this goes wrong. And then Aoi turns up. If Aoi got away, he could also be the replacement Gatou hires once Zabusa changes sides during the Wave Country mission.
I like that team picture, btw.
D for Drakensis
You're only young once, but immaturity is forever.