I have never seen a Koreeda film but he sounds compelling -- which movie would you recommend for a first-timer?
On the arthouse circuit, I think he's best known for After Life, which is a bit more challenging (honestly: I found it a bit dull) but worth biting into.
Do you know that pang of melancholic joy-and-regret you feel after you've had a wonderful day and you know no matter how much you and the others involved try, you can probably never quite recreate that magic a second time? Grateful for the memory you'll always have, yet at the same time sad? That's how his movies feel to me, where I'm often both happy and sad I've seen them. It's pretty damn great when a movie can do that.
So maybe I would enjoy his other movies, if you liked them!