For what it's worth, I think Hatori's current story would work fine for an introductory adventure if the corrupt emperor had something the characters wanted. Maybe they hear that he's come into possession of a chart that will get them through the dangerous waters near the Island, or his capital city is just above some ancient ruins that they need to study, or something like that? That way the characters have a reason to fall into Hatori's plot beyond 'some stranger asked', and Hatori and his fellow samurai can be introduced as allies of convenience?

Not my call, of course, but I think that's the best of both worlds. It would probably be a longer adventure than some of the other ones, but by that point I think it would be fun to have the assembled characters tackle something really big.