4.0 New Game Mechanics [StepLeaks]

Keep in mind that this appears to be a new leaker, so take this with a very big grain of salt. Their first post was a leak of MiHoyo offices from the leak dump a few days ago.

A new feature that will be introduced on 4.0, is be the ability to collect seagrass, those will be present underwater.

New animations and models regarding fishes will be introduced to the game on 4.0.

There will also be a new natural mechanic to help you move underwater, they are called "Ocean currents" and will give you momentum while swimming on the depths of Fontaine.

New tools regarding the artifacts system will be added on Fontaine.

Source