Would love to have an optional feature for a book to add a deadline.

Reasons for this can differ. For example I read a lot of books from my library so setting myself a deadline by when I have to finish a book in order to hand it in at time would be amazing.
Also if you are doing a buddy read or a book club with a deadline a feature like this could be helpful.

Optional to show the deadline on the currently reading section on the start page if I have only a few days left. If I still have longer time left it can be hidden in the book detail page.