It's rumored that Android O's feature set will include new notifications, but more specifically that Google will be looking to revamp the notifications in some way. There isn't really much detail here in regards to how things will actually change, but according to the latest rumor about the next version of Android notifications are due for an adjustment. That could mean another new visual appearance or it could just mean that Google is going to alter the way we interact with them. Or perhaps it'll just mean there will be some new features added to the notification functionality.
New notifications seems to be just the tip of the ice berg though. Android O (Android 8.0) is also rumored to add in a bunch more stuff, such as badge icons for notifications. What this would do is allow people to glance down at their app icons for things like messaging, emails, calendar, and more, and if there are any new notifications relating to those apps, then a badge would denote how many notifications are unread so users would be aware of how much they have to catch up on. This isn't really anything new even in Android, as third-party launchers have supported it for years, but stock Android has never included badges, and it seems Google may finally be incorporating the function with Android O.
Notifications aside, another new potential feature with Android O could be picture-in-picture. If you have an Android TV device then you may already be aware of what this would look like on Android-based hardware, as Android TV already offers this feature to owners, now though it may be making the jump to Android mobile devices. For those that have a Pixel or Pixel XL, a feature that Android O is rumored to include called "adaptive icons" may be familiar. While there is no way to know if the two are one in the same, some icons on Pixel devices, like the calendar, will change based on certain criteria. The calendar icon for example will change based on the day, and today it's March 17th so the calendar icon shows a number 17 on the icon to match. Also rumored to be included in Android O are restricted background activities from apps, and a floating toolbar for smart text selection that will integrate somehow with Google Assistant, though there isn't really any details on these two features. For the most part the features talked about so far are all things that can be seen and visually interacted with in some way, but there are potentially a couple of under the hood changes as well, like improvements to the MediaRecorder API. It's also said that Google is planning to add in more Enterprise-related features, though nothing specifically was mentioned at this point. However, Google is taking a more serious approach to Enterprise with Android as of late so it would make sense to see new and improved Enterprise features in the next version to keep that momentum going. As it gets closer to Google I/O there is a chance that more details on Android O will spill out and this is all the more likely if Google ends up releasing another set of developer preview software for this next version of Android, but nothing will be concrete until at least Google I/O.