Spell Alerts support corner location, namely TopLeft, TopRight, BottomLeft and BottomRight.

Problem

However, their current implementation displays a small square with the width of left/right textures and the height of the top/bottom textures.

Not only the textures are small, but textures look weird because they were not designed with a square shape in mind.

Because of that, the corner location are simply never used by the addon. Neither are they used by Blizzard.

Solution

The solution would be to get rid of the square textures and display rectangular textures in diagonal (e.g. 45 degrees).

Texture locations would still be the same, e.g. TopLeft would still exist and would be re-used to display the new corner textures.