Actually we have a very good terrain for sledging even in high altitude. Thus you can enjoy ski / snowboard touring in spring!
We have longer day time and milder weather in late March, April and May so that you can go deeper and further. It's not as well-known as powder and you can get all the best by yourself. You can sledge even from top of Mt. Fuji!
There are much more options than one in powder season and this is what I really recommend to you!
Late March - May
Mt. Yotei, Rishiri island, Shiretoko Peninsula, Asahidake, Niseko, Oputateshike
Mt. Fuji, Hakkoda, Chokai, Gassan
Japan North Alps - Tateyama, Tsurugi-dake, Yakushi-dake, Suisho-dake, Yarigatake, Hotaka, Hakuba, Yukikura-dake, Renge onsen
Japan South Alps - Senjogatake, Ainodake
Japan Mid Alps - Senjo-kar