It would be REALLY great if we could have elements in the global XML that allows the mod creator to turn off certain parts of the TTS UI for all players.

The reason for this is that a lot of mods don't have a need for certain TTS interfaces. For example, you might only use the hand tool, so showing the other tools is useless (and their use could be even discouraged!); the top buttons might not ever be used; the notes in the bottom right might be left empty, etc...
Therefore hiding these interfaces for the players would give you more precious screen real-estate as a mod-maker to make a good-looking custom UI that is placed in a sensible position. (I actually wonder if one of the reasons we're seeing so few Custom UIs in mods currently is due to all the common position for UI elements, like the corners, being already used by TTS)

To turn off TTS interface, you'd add an element on the Global XML that would deactivate a specific TTS interface, such as the tools, or the top buttons, or the notes, or the player list, or maybe even the chat! When you launch a mod that hides a TTS interface, a message would pop up in chat (or on-screen similarly to broadcastToAll if chat is disabled), telling you that "this mod has hidden [list of TTS interfaces], and you can bring them back with F11".

F11 would now cycle between
  1. Mod-determined interfaces
  2. All TTS interfaces (hiding custom UI)
  3. No TTS interface
instead of only cycling between [all TTS interface] and [no TTS interface].