I have played for years NDS games with Desmume (the NDS emulator) using as LCDs Layout the Horizontal one.
I think it would a great idea introduce an Horizontal mode also in Citra.
I know that already exists a Large Screen mode but it is not the same. Using Large Screen mode the first screen I think is too much large and the second is too small.
I think there’s need to implement a new Horizontal mode with the first screen larger than the second, but not as large as in the Large Screen mode. Almost having the same dimensions, respecting in a better way the real 3DS screens proportions.
Also the color of the background near the two screens should be replaced from white to Black, to relax eyes.
I think these changes are very important to enjoy 3DS games on a PC with wide monitor (16:9, …) and resolution such as mine 1440x900, etc.