According to PWInsider.com, WWE held a big creative meeting while in New Orleans this week to begin booking the Royal Rumble match and other aspects of the PPV on January 24th.
As of this past Monday, none of the rumored top New Japan Pro Wrestling names had signed WWE deals, however there are definitely ongoing talks between the company and AJ Styles and Doc Gallows. As we noted earlier today, Shinsuke Nakamura will be heading down to the WWE Performance Center in early February to begin medical testing and to workout in the ring. It is likely Styles and Gallows will also have to undergo the required WWE medical testing before they can sign deals.
Karl Anderson is another rumored NJPW name in talks with WWE, and he confirmed on Twitter back in the summer that his NJPW deal expires February 1st.
So while it is a possibility that the rumored NJPW stars could debut in WWE as early as the Royal Rumble match, it might take awhile for all of the pre-contract steps to be taken before any of the talents can officially sign with WWE.