Android O (due out this fall) will have version number 8.0.0 (source) but will it have so much changes that it requires changing the version number? I think no but hey, current operating systems are so mature so it is very hard to rewrite everything. Google has to polish the good things and rewrite the bad things but the bad things list isn't really very long, in my opinion.
What will be new:
- Notifications Channel - now developers can group their notifications, set names for them and change behavior (sound, vibrate etc.). For example for Twitter you can mute "Like" notifications but leave others. All this will work out of the box without any additional work.
- Notification Badges - there will be cool red dots with number near your app icon. Something available in iOS 1.0 (or iPhoneOS) now is available on Android. Your client won't be upset when on Android they don't see similar red dots ;)
- Notifications Snoozing - You can snooze your notification and read it later. Similar to Google Inbox. We will see if it is game changer or a not very important feature.
- Autofill Framework - I think this will be killer feature. I am using Dashlane for saving my password and generating a different one for each website and finally I haven't use custom browser with this feature. Currently this isn't implemented but this is already done in LastPass and 1Password. Big improvement for security because now users won't have excuse why they don't generate strong password.
- Android Picture-In-Picture mode - this feature is already available on Samsung device and Android Tv is available for everyone.
- Downloadable fonts - your UI/UX designers will love this. They can use every font and your implementation will be very simple instead current implementation with attaching them to apk and use library like Calligraphy - context wrapper. (source)
- Adaptive Icons - user can choose icon shape and your app's icon will change to it dynamically. (source)
- Multi-display mode - you can show your app on two different screens. It will be very useful on workstation or notebooks so maybe Google is planning to make Windows competitor on PC? Who knows.
- Cached Data - your app can save all data in cache quota. It is better then default cache because Android will delete the oldest files instead all of them. Really good improvement.
Android O has many more improvements – for the whole list you can open Android Developer page.