Unless New Japan starts booking women on their big shows, there's nothing left for her to do in Japan. Truth be told, she outgrew the joshi puroresu scene a long time ago. She had two seperate year+ long reigns as champion, headlined hundreds of shows, held all the belts simultaneously, beat everyone, carried the company on her back for years, won the Tokyo Sports award 3 years in a row, etc etc. It's only so many times she can fight Mayu Iwatani and Meiko Satomura in front of the same 1500 old Japanese men at Korakuen Hall.
I'm speculating but I think she would have retired by now if the WWE opportunity hadn't materialised, and I think she's done after her WWE run. My gut feeling is that she came to America to experience some big event glamour, show her skills to a worldwide audience, maybe have her "WrestleMania moment", then call it a career. Joshi wrestlers (especially the non-dykes) generally retire young, it's rare to see them going deep into their 30's.
As if Io would willingly go from having 10k people going crazy for her at TakeOver to scrapping with Paige's mom in front of 175 rabid feminists in Pro Wrestling EVE