The HUD system implemented in SL has proven to be an extremely useful feature for extending avatars' abilities. Desirable HUDs quickly pile up, and because they take up viewport space, the interface aspect to them tended to be more of hinderance, which is why most HUDs implement a minimize button to prevent that from happening. The word HUD fits less for what it has been actually used as, ie more of extending functionality and providing an interface for controlling it, and less layering additional information on top of the viewport like RL HUDs.
I therefore propose a few things.
1) Ditch the word HUD and just call it tools/plug-ins/extensions/addon/etc.
2) Apply a similar design pattern to how the chrome browser implements plug-ins, with icons of plug-ins listed as a row at the top of the viewport, and only once clicked do they reveal an interface for controlling the plug-ins functionality, if at all needed. For vr the icons can be placed on the menu that appears on the left arm, such as when calling the atlas browser.
3) Since 2D is the ideal for control panel interfaces, it would be nice to have the plugin interfaces implemented with html/css, since that is the most well known and a flexible inteface for making interactive and engaging 2D experiences, and is easy to learn.
The main point being that it is about attaching scripts to avatars that just add functionality, and less about actual HUDs.