A new mirror, big enough to cover the second, smaller cut out will change the overall scale, giving the fireplace a grander feel. A mirror in a metal frame or a material like the horn shown compliments the woods in the room, and ties them together.
One step further, a look I’m loving right now is a pendant, either a chandelier or hurricane hanging in front of the fireplace. Depending on the mantle width, a few accessories, candle sticks or even the pieces up there now will look completely different layered in front of a bigger mirror.
Hope that gives you an idea Beth!