F10, which is the default for toggling the UI, is great but not very useful. There are certain things we pretty much have to be able to see. One example of this is the character (ALT Y) and the Group window. And probably a single chat window. However, there are a LOT of windows we don't need. A number of the chat windows, the spell bar, most of our hotkeys, auras, window selector, buffs, songs, etc. If we had the ability to customize the Toggle UI display feature, we would be able to select the key windows we want to be able to see 100% of the time (our HP, mana, pet HP), and toggle off most of the windows we only need situationally (spell gems, hotbars). This could greatly increase the usefulness of the toggle UI feature and greatly increase the immersiveness of the game.