The Kindle actually has features dedicated to audio reading of it's books, at least for some of them. It's an author opt-in; meaning that only books that the author (or publisher) approve get the nice reading options.
The ability to listen to book in iBooks is a function of the Accessibility features, a general iOS feature that lets you have text read out loud in almost any app. Since it's not specifically for iBooks I wouldn't expect any new features in Accessibility to be designed specifically to improve the iBook experience.
That said, Apple could decide to add the feature to iBooks at any time. The biggest hurdle is convincing authors that it is worth while, since it tends to eat into the fairly lucrative audio book sales. Amazon has a lot more pull with authors and publishers. If Apple does add this kind of functionality to iBooks, I expect it will be a substantial improvement in features over Accessibility more generic set.
Apple does have more voices in OS X, so it would be easy for them to add more; however it looks like they provided the best of the OS X voices for iOS, so other than for variety (and nostalgia for us long time Apple users) there would be little point.
What Apple actually plans to do? Who knows. The don't typically announce new features and products until they are ready (or nearly ready) for the market. Sometimes, if they are minor features, you won't hear about them until some tech journalist stumbles across it.
As a side note, the iOS 8.4 updated that came out yesterday moved audio books from the Music app to the iBooks app. You can find it as a new built in category; Audiobooks.