Here is one idea.

HI -- There are some amazing molded concrete surrounds now that look like fine stone . You should do a search and see what comes up . Just the raw concrete is very on trend right now also if you want a more industrial or rustic look. Maybe check out antique shops or architectural salvage places . You might find something interesting like a Victorian or cast iron surround or something stone from a French castle .

- It seems like you might want it to be a little rustic, especially if you're thinking about a rough-hewn beam.

What kind of fireplace are you installing? It looks like the clearances are a little tight with your existing cabinet work.

Can you take couple steps back and take a photo of the the whole wall so we can imagine what some stone and a beam mantel might look like with everything else?

This project should be a nice update for your room!

We are looking at the Heat and Glo Slimline 550 trs.

Here are the fronts that we are choosing from. We're leaning toward the Aero Front. I like the Chateau, but don't want the front to take away from the stone(?) surround. I envision that to be more the focal point. I don't see any benefit to having doors that can open. What color??? Maybe that depends on the stone chosen?

- I'm a little concerned about the clearance to the bottom of the shelf, but you can check the SL-550TRS manual for that. You might have to raise the shelf a little.

What about considering a stunning stone (quartzite or onyx) slab to be the focal point? Something like these:

Concrete is very versatile and it looks great with wood, stone, glass, and metal. A quick picture search should give you plenty of ideas.
I'll attach a couple of fireplaces we've done recently. There are better photos from after completion on our Houzz page but this is all I could access from here.
One uses a wood mantle and matching built-ins. The really tall one used a slab of granite set in the middle for interest.

The computer or web site was acting up earlier. The comment with pictures was loading when I left. It looks like only the comment went through.
I'll attach a few pictures now.
The last one (flat/flush with stacked slate) was still waiting for a glass shelf with a radius edge.