Other than just more expressions hotkeys, I think the feature that would be the most useful to the most amount of vtubers would be controller based eyetracking. Where the model looks in the direction a controller's right joystick (which is generally used for camera in games) is being held, like when holding left they look right. But when the stick is let go, it slowly resets back to looking forward. This would emulate the behavior of someone with live2d facetracking really well if put in place.