Aren't there law's that you can get japanese citizenship.
If you spent xy years (without leaving) there or when you married someone?
(tough it most times cost money (alot), when they want you to do any tests. Checked that for some other countrys, most time around something like this.)
You can get permanent residency, that's one thing I'm not interested in getting. I'm happy enough with my business visa. 8 years at a time is more than enough for me.