Forget 4.X. What big features are you looking for in Android 5.0?
Here's my (probably not exhaustive list):
- As notifications function in Google Hangouts, they will function for every app. That is, if there's a web component, dismissing notifications on one device dismisses them on another.
- Building on 1, a notification extension for Chrome so that you can respond to texts and emails, and see and dismiss other notifications.
- DEEP Google Voice integration. That is, similar to iMessage, a messaging protocol that is totally seamless from the user's perspective. The Messaging app would use Hangouts/Voice as the backend and only send actual SMS when that backend is unavailable (for example, when messaging iPhones that don't have the Hangouts app installed).
- Similarly, to actually unify all the messaging apps on Android (they're not there yet). Messaging, Voice, and Hangouts are one app (perhaps just called Messaging) that supports SMS, MMS, regular text, video calling, etc.
- A backup system similar to Titanium Backup that auto-syncs to Google Drive.