Mirrors would be an awesome addition to the game, mainly after player modelling is created. They would be good as thinner surface plates (similar space usage to minecraft's slabs) that can be placed on walls, floors and ceilings/rooftops giving off a reflection from the mirrors point of view. I've seen it done before on map mods for games, but not in Battlefield 3 by EA. I have no clue how to create it, except for the fact that the mirror model is assigned a texture, then the reflective property is assigned by a shader.
TL;DR: Mirrors would be a good addition as a slab feature.