Xiaomi has released teaser images of its new MIUI 9 features and a teaser video of the Xioami Mi 5X . As we know their upcoming event is scheduled on 26 July that is tomorrow where they will officially unveil Mi 5X and MIUI 9. The company has already released official images of Mi 5X few days back.
Some notable MIUI features are:

No Boundary, Color Fantasy, and Cool Black as new default themes. The dock icons at the bottom now do not have texts below them giving a neat look to the home screen.

New lock screen will let you long press home button to activate torch as it will unlock the phone. Mi Home, Mi Remote, Mi Pay, Transport Card, and Torch other useful and commonly used shortcuts can be accessed in one place.

Split Screen Feature introduced in Mi Max 2 and MIUI Beta ROM for Mi 6 and Mi Max 2 will be released for Mi Devices such as Mi6, Mi MIX, Mi Note 2, Mi Max, Mi 5 and Redmi Note 4X and more devices

Child Mode, WiWide (provides public wifi networks to stores and restaurants in China), Contact Card, Showtime (Caller Profile Video) and more will be removed to simplify and optimize MIUI ROM