After painting I would perhaps try a couple of things on the hearth....plant, basket, sculpture...? I would be inclined to NOT draw attention to the mantel.

I would paint lower cabinet doors black and insides of bookcase black. Add a couple simple white and gold accessories along with a group of books light backs. Add simple look on mantel on side of TV. Add black woven basket for toys beside fireplace. Add stack of birch logs to fireplace for decorative look if you don’t use it often. Do not put personal photos on this wall. Better in gallery arrangement in master br or in hallway. You can enjoy more when you can get closer to them. Guests aren’t interested in your personal stuff. Keep it simple since your sweet baby will be running around before you know it!

for starters, I would paint them both White. the one above the TV will blend with the furniture, I'd leave it empty. above the fireplace, you could add a shelf of a wood on top of the sheetrock opening (something you already have in the room to warm the space) and add natural filler millwork to build up your mantle to a fuller depth. Consult a finish carpenter or cabinet maker for skills.

Keep it simple. A picture that is horizontal versus vertical will be better for the mantle. One print could be sufficient but you could add a small bit of greenery and a candle. I also like the pick with just a plant on the hearth and nothing on mantle.