I've got a soft spot for MMORPG UI and would like to be able to client side script custom RPG UI with my own theme and style. I'd want a great deal of artistic control over how the UI looked and also be able to do hover effects and other things that feel 'native' that standard huds just can't provide.
I'd want to do a chat replacement in rpg style, We'd probably need some way to do text input UI and maybe also let us send stuff into local chat from said input/receive local chat in the client side script? I suppose lsl scripts could act as local chat relays though.
It would be cool to essentially just temporarily get rid of the entire SecondLife UI during a game and replace it with a totally different game specific one, although this would probably need some clear easy way out for the user.
I am not sure how something like this would best be implemented. I am simply stating the desired goal for now, and not presenting a solution. This is just food for thought.