J.K. Rowling tweeted minutes ago that she just finished a new novel and it will be published next year. Her tweet is a reply/quote to another tweet asking for “Strike news”, so we can assume she is referring to the sixth book in the Cormoran Strike series.
New book finished, out next year 🖤 <- name clue! https://t.co/e7P2uOl5ND
Even more, the name clue, which shows a black heart, coincides with the clue that was discovered back in April and published in HogwartsProfessor.com: The Ink Black Heart.
Update: Moments later, Rowling tweeted again to confirm the title of the book:
Ladies and gentlemen, we have a winner 👏👏👏 https://t.co/XDAja9JvKr