I just did a search online for new voices for VoiceOver on iOS 11, and found that we'll have one, at least, one exciting new voice. The new American English female voice for Siri can be used with VoiceOver, and she sounds better than ever! I think she is the closest thing we have matching Ivona's TTS engine. I personally am really looking forward to this one!
Can't wait for next week, so thought I'd share this with you guys.
Meanwhile, take a look at this article: