The reason why most people think it's Asuka because she left this message in her blog?/twitter?, on 20 August.
After translation, it says it will be debut in September!
The weird thing is the message was removed afterwards. You won't be able to find this message in her blog now.
Now the million dollar question is, why she deleted that message? What did she mean by mentioning the word "Debut"? Since she is already a gravure idol for many years, then it's definitely not an IV debut. So a JAV debut perhaps?
Although many people hope it's her, she still got some TV appearance scheduled in next couple of months. She also will show up in a new movie in 2017.
Is she really Asuka? or somebody else? We can only find out on 1 Sept.