I would like to update this fireplace or figure out how to decorate the huge nook
Hard to tell without seeing the rest of the room.
So hard to help without seeing your decorating style in the rest of the room.
I love that fireplace! I'd put shelves and fill it full of plants or one big plant.
Beth: to further Pat's idea... paint the nook in a neutral color of one of the fireplace stones. Then floating shelves! Win!
The stone is great but the nook is an eye sore. Maybe since the stone has a natural and rustic look, a black and white print, the same size as the nook, of a close up of an animal's face. A horse, elk, bear, cow, moose, or any large animal's portrait (not too cute) is always striking and fits with the stone look. Many sites that sell wall hangings sell them.
PS. Replace, or paint, those outlet covers to match the color of the stone.
My suggestion would be to paint the niche a neutral color that blends as well as possible with the stone. Get a couple of tall elegant vases in a color you love and add bamboo sticks or ting ting to them to draw the eye upwards. Arrange the two vases so they cover the electrical and cable hardware. It could be the focal point of the room and a serene place to focus upon. I think the stone is absolutely beautiful!
It is hard to answer your question without knowing what style you like or are looking for.
I feel that nook is screaming out for a great little water feature. Love the sound of running water indoors and out.