Frequency of use on-screen keyboards are already available for switch users within the Grid 2. This Grid bundle applies these principles for an eye gaze user. Most commonly used letters or functions are in the middle of the keyboard, less common letters and functions are placed around the edge. Where prediction cells would be beneficial, these are in the centre of the keyboard. In principle, this should mean that the user (after they become familiar with the unfamiliar layout!) should have less scanning of the screen to do. Bringing the prediction cells away from the peripheral vision of the user, may also help with the efficient use of prediction, and hence the speed of use. Try it and see!
Some eye gaze users also report preferring darker backgrounds and less contrast with cell styles. Simply changing grid background colours and cell styles can help with this. Look at "Star Talker at Night" as an example.