I would cover the corner with stone and add a more significant hearth as shown in following inspiration pics. Yes, yours will be narrower on the sides of the fireplace, but it will still look fantastic.

Here is another example of a corner like yours finished in stacked stone, but only part way up the wall with a thinner reclaimed wood mantel and a miter cut to be flush with the side walls.

Thank you guys so much these are such great suggestions!