There are quite a few problems with OP, which new decade are we talking about? Are we talking about the 2020s which is... 2020-2029? Then either you restrict to girls debut on or after Jan 1st, 2020 or not restricting debut date at all.
Most (> 50%?) of JAV are featuring idol(s) whose career will be less than 3 years. So it's very interesting (and common) to talk about idol of the month, idol of the year. But idol of the decade... hmm tricky. I guess we could sort of kind of invent some conceptual idol of the year. Something like...
1. she is active for at least 5 years (it's pretty hard, 4 years would be much more inclusive, but how can we give the title "xxx of decade" if she's not even active for half a decade?)
2. more than half of her career overlap the decade (say 2020-2029), so if her career is 5 years, she might be active in the decade for as little as 2.5 years
3. her career peak occur in the decade,
Hmmm... pretty hard... try to imagine come up with a short list of AV Idols of 2010s, but now do it for a decade that just started. Anyway let's try to stick with the spirit of the topic:
If we are talking about new new new girls, hmm we are only 20 days into the new decade, it's kinda nuts to talk about idol of the new decade. But a debutante that really got my eyes is
Kyoko Akari 朱莉きょうこ, not even 19 and a half years old. T169 / B101 (I-cup) / W59 / H93, debut with S1 just yesterday. Not the most beautiful face or figure. But something about her caught my eyes: she's big. Not really very tall, but her shoulders, hip, long arms and legs, plus very nice big tits and ass. Not a giantess (who are usually ugly), but a pretty girl who give a giantess vibe. Not sure yet, hope she will have a full career. (you never know, such young girls tend to do a short stint career and move on with their lives) Oh another problem is her bone structure looks like the kind that would turn chubby easily. So she looks mesmerizing at 19, but could easily turn into a cow by age 21.
Ena Koume 小梅えな just 20 years old T162 / B103 (J-cup) / W59 / H88, career is not yet 6 months. Started with Moodyz in their premium label but now seems she is freelance and both increase her output (8 full length solo vids in 29 days) and giving many major studios a sample of her soft supple meaty body. This one is more promising as a lifer.
Finally on Ruka Inaba... personally she's not my type, a big reason is she's on a new vid almost everyday. But of course that doesn't mean she can't become a "idol of the decade". She is definitely committed to be a lifer, but a big question is how long the fandom will support her.