Some phones in the Android One program should also have a decent chance of getting it quickly. Everything else will probably get quite a few of the features from the closed source parts of Android that are shipped by the Play Store anyway.
Android OS features can't be updated from PlayStore, other than the WebView.
What are you calling "Android OS" in this context? My understanding is that a lot of APIs that used to be shipped with AOSP and be updated with each new Android version are now shipped in the Play store to avoid the mess that is Android fragmentation.