Hi, unfortunately (or maybe not
) Addons are not able to change any enemy/friendly textures of the units in the game.
You are able to add some Controls (e.g. a rectangle box) to the Ui, and if grouped also use the position api of the group members to add a ray of light (Light of Meridia) or something else to the group members. But everything else, like add a marker icon on enemies head, change the look or size, change the color or the selection frame, or change sounds of effects or the effects of skills, is not possible.