There were a lot of brilliant guesses, and the funny ones are almost uncountable... Until one Big Daddy guessed it right: Lethal White. This initiated some more frenzy among the fandom and I am not surprised because the Cormoran Strike novels are such a very good and enjoyable read. I really have to satiate my hunger for the said book and only creating fan-made covers for it will I ever have some mental peace.
Check out the fan-made covers I created for Legal White by Robert Galbraith (a pseudonym of J.K. Rowling). I did my best to be true to the original designs of the first three books in the series.
No official release date has been set yet, much less the teasing synopsis. Robert Galbraith is still writing and working hard on it. Only some chapters have been finished and there will be more drafts after that. Hopefully we can expect a late 2017 release in the latest.