I know this has been requested by many people, so I am surprised to see no Wish List entry for this. I would really, really like to see the /viewport command from EQ1 introduced into EQ2. The letterboxing we currently have is simply unsatisfactory for me, as well as many others.
For those unfamiliar with the /viewport command, what it does is it resizes the rendered 3D world to specified dimensions and then moves starts rendering it at a given pixel location. This makes it so that you have black on the screen outside of the rendered world, while the rendered world centers the camera in the middle of it, regardless of it's location on the screen. The letterboxing does the former already, as it does not render anything that appears under the black, but we cannot resize it horizontally. We also cannot move the location on the screen, therefore causing a percieved distortion of the camera location if we border one side but not the opposite side equally. Sorry if that was too complicated, for some reason I just can't dumb this down. My personal example below might help.
I run EQ2 in 1280x1024, and I prefer all of my gameplay UI elements on the right side of the screen, with all my chat elements across the bottom. Currently, if I did this, the center of the camera would be very close to the bottom right corner of the viewable area not taken up by UI elements, meaning my character is completely off center from where my instincts tell me he should be. With the /viewport command, I would be able to tell the game to start rendering the world at 0, 0 (the top left corner) and render it at 1024x768. The game would than render the world at the desired resolution, leaving a large black border across the bottom and right side of the screens, 256 pixels wide (1280-1024=256, 1024-768=256). The world would be rendered just as if I was running EQ2 in 1024x768, meaning the center of the camera would be at ~512x384. I would then be able to put my whole UI across the black border without covering up any of the game world.
I believe that everybody who thinks this should be implemented should /feedback it. That doesn't just apply to this, but to everything you think should be added. Hopefully the more popular requests flood the feedback enough that they would be hard to ignore.