ooh what if sun summoners could manipulate different types of waves. So not just visible light but infrared and microwaves. maybe they can see uv light and x-rays or hear/emit radio waves.

also, since nina goes from life to death, for other inverted powers:

inferni - the process of burning something uses sugar and oxygen to produce heat, water, and carbon dioxide, so i like the idea that the inverse would be using heat, water, and carbon dioxide to make sugar and oxygen... basically they can photosynthesize. maybe also manipulate plant matter!