Niseko is home to very tame terrain. But it is just a resort on a hill so forget about it. More interesting is the mostly neglected surrounding areas in Hokkaido (the island on which Niseko resides). It has the potential to offer up some seriously rewarding and relatively safe touring terrain with seemingly limitless Utah-like powder: all uncrowded and yours to enjoy. Sounds good. But in the world of steep and long descents it doesn't really rate. And Niseko resort itself is flat and unchallenging
The stretch of mountains near Nagano city (90 minutes from Tokyo by train) is home to great quality snow, usually lots of it and with real mountain terrain. The ski resort valley/village area called Hakuba is the epicenter... but there are way more mountains in that alpine range than those well known ones along side Hakuba. I know some crew there.
Also Yuzawa is worth checking, same easy access from Tokyo, good BC scene, some steeps but perhaps not as rugged as Hakkuba area. There is a splitter there that used to hang on this forum. He really knows his local stuff.
I used to live in Japan and am most likely heading back to live there again before next season.