Niseko is super busy, the lines of people hiking up the peak are ridiculous, verging on dangerous with that many people out there. The main freeskiing zone in Nis would be Strawberry fields at Hanazono, it has a lot of step downs which are fun. The best pow skiing inbounds would be in the avi controlled zone under the Niseko Village gondola. It'll cost you an extra $20 or so but it's worth every cent - probably the steepest, deepest and quietest area you can ski. If you're looking for pow head across to a small resort called Moiwa. It's a free 20 minute bus ride but the place is empty and has powder for days - it's not very steep though. If you're going to Japan to ski park... Don't. Go to Breck. They make these strange table top jumps that you have to carry a shit tonne of speed into for not much pop - although I saw quite a few of the locals throwing down with 10's, rodeo 7's and cork 9's.
I don't know about any other mountain in Japan - apparently Rusutsu is pretty quiet and has powder for days and Hakuba is a lot steeper, but I'm not sure about the park setup.
If you're going to Japan and spending most of your time skiing park, I will make it my personal mission to find you and slap you into next week. Don't do it. Expect to wreck your legs from half a day of skiing pow, and maybe session the park on sunny days with no snow or any afternoons where you've got a bit of extra energy.
Oh yeah... Niseko also lights up every night, so you can literally ski 8.30am till 9pm-ish. One afternoon it was dumping so hard that we were skiing waist deep fresh lines through the trees all night. It's the most phenomenal and surreal feeling, but holy fuck I'd relive it again and again if I could.