New art will be released with the coming of this book.
Shannon Messenger's contract only lasts until book 9, but she can always renew it, so there is a likely chance this will not be the last book in the series.
Shannon Messenger hinted in an interview that she didn't think she'd be able to wrap up the series in only one more book, so there would probably be a Book 10, although no official contract had been drawn up. It looks like book 10 might be the last book in the series 
Book 9 will come out sometime in November 2021 
Jason Chan will be creating the cover art, as he has in the previous books.
Shannon Messenger announced Unlocked on February 19, 2020, so we can expect to receive more information on Book 9 around the same time in 2021.