she never really had any interest in the industry just the money, so I see her staying Vanilla and getting paid ...
Also what do you think about Shoko, and Yua? two other girls that came in with big hype and a good following.
Hmm here we go, wading chest-deep into semantic quagmire...an exception like Kirara Asuka who debuted a decade ago and can remain on S1 doing relatively normal shit on a big label
To me, Kirara Asuka is another perfect example of what I call standard AV career or what you call "mainstream", sure she's not doing enema or bukkake but she's nowhere near vanilla as Rion is now (and always been). Now maybe you would disagree with me... but seems to me Kirara is even more "down to earth" than Julia. Put it this way, if Rion can follow Kirara's career track and stay with S1 for few more years, I would be happy enough.
Problem is that Rion seems to be unable or unwilling to commit to the standard top idol career. It's showing in her work, and I suspect it's starting to affect profits (especially it seems her average production cost is higher than the typical top idols). She's milked (ahem) her amazing asset for 3.5 years and she's now still at the same point other fresh faces+good figures after about 1 year. I'm predict a noticeable change in her career within 6 months. Either mainstream or retire. (but then ... what would she do? She already unretired once, why did she do that? If she can't figure out an exit strategy it may end up badly for her)
As for Yua. I think there's a chance (maybe a small chance) Yua will turn out to be a groundbreaking, genre-bridging superstar. She was not talented and disciplined enough to be successful in AKB48, and in AV at best she would be another Yuma Asami. But perhaps she can bridge the two that happens not even once a decade. Maybe she can make regular appearances in small TV roles and other celeb jobs while keep making "classy"/"vanilla" AV. The AV lends her sensational cred in the non-adult media circle (but can't do any hardcore) and her ongoing non-adult celeb status is needed to keep her AV sales (and her own income) sky high. A wet dream perhaps, but I think she's unique and has what it takes.
A more reliable fallback, my wishful thinking: she becomes Yuma Asami: one vid per month, top idol with strong professional altitude and acting skills. Bad scenario: she can't figure out what to do with her life nor learn to be content with what she has, end up in drugs, prostitution, debt and prison.
Shoko: it's too early to tell, she can follow any of the typical career paths we already discussed. Compared to everyone else, she's still fresh enough (only 9 months) that she can do vanilla for a little while longer.