is this the current look or before the window was covered? its not really too bad, considering your furnishings are dark.

built in cabinets with interior lighting would lighten up the space ref pic for inspiration

This is the current look, the window was over to the right; this is where the opening into the sunroom is now.

The built in cabinets with lighting is a great idea, thank you!

Think hard about painting the fireplace, most people I have met ended up not liking it, and it can not really be undone.

To be honest, with all that dark furniture changing the fireplace won't significantly make your room feel brighter. If you need to keep the sofas, consider new tables. Also, cream or white throw blankets on the back if the sofas would add a lighter look.

I think with the window gone, your fireplace solution may be quite costly and might not solve the problem.
I would think, you might first either rethink your furniture to be smaller, "lighter" pieces...think white duck slipcovers or blue and white or bright primaries...and the room less "full"...
Or you could embrace the dark cozy aspect and keep the dark furniture, add some more dark/jewel tone elements, and enjoy it as a quiet dark TV room.

i'd love to see you create a FP surround from white wood and mouldings, keeping the brick at the opening only. then add some white built-in units to flank both sides of the FP, similar to the above suggested pic. you can create a similar look for much less $ by using books shelves and making them look custoin with mouldings. ;) seriously consider adding either some pot lights or a ceiling fixture. a new wall color would be great also. with your furniture, i'd go for a light, gray-blue. look at benj. moore woodlawn blue. it's a great room brightening shade. aim for a palette of the gray-blue, dark brown (furniture), whites and silver toned accents and a fresh green, or an aqua accent color. adding a mirror and using art frames in silver, lampshades that are all white and drum shaped, and a few bright pops of color with pillows, a throw, more large scaled wall art, etc will really brighten up this space. you might even consider painting the tv cabinet too.

Here are some inspiration pics.

Adding mirrors to the sides of your fireplace would bring in light. Reflecting from your sun room light.

You can do this in many different budgets. Whether you white wash, keep as is, or sheet rock over it, to give you the look you want.

I too like the darker paint idea. lightening a darker room with darker furniture can be accomplished by added metallic highlights, brass light fixtures, mirror accents, they do not need to be large, metal wall sculpture that is brighter metal. silver and/or gold picture frames, metal sconces and candlesticks. Another help are a couple of larger pics with lighter back grounds.

Or you could add a large wide work of art above the mantle that is light. Then you would not need to white wash it.

Thank you everyone!! I really appreciate the feedback!