Hallo nach Muenchen!
There are certain things that are especially beautiful in winter.
I recommend visiting Kawaguchiko because you can take great photos of Mt. Fuji. With the cold weather, the sky is usually clear and Mt. Fuji rarely hides behind clouds then.
Definitely visit the "snow monkeys" in Nagano who are bathing in hot springs. I also went there in winter a few years ago.
And if you really go, then you should also check out nearby
Matsumoto Castle, one of the very few original castle structures left in Japan!
Hokkaido is nice, but personally I'd recommend it rather in summer than in winter unless you go to the "Yuki Matsuri", but that's in February.
The Tohoku region is extremely beautiful, but has a lot of snow in winter, so it might be a hassle to travel around.
The Chubu region (Nagano, Gifu, Niigata) also gets a lot of snow, but Shirakawa-go is nice in winter!
I also went to
Kyushu in winter. No snow at all, lots of palm trees and beautiful beaches.
You can see my winter vacation Kyushu trip
here.
There are also "special" things to see in Japan like
Battleship Island,
Cat Island,
Rabbit Island or just go and see
Tama, the Station Master Cat.