A floor vent can be re-designed to vent out the front, from a section of the toe-kick of a built-in.

IKEA will have lots of options that may work for you. What is your budget?

Hoping to stay mid range, probably around 4-6k

Can you show the rest of the space? Are you sure this is the best spot to create a focal point with tv and fireplace and built ins?

I would not go built-ins next to the staircase. I would choose some nice furniture pieces with legs so you do not have to worry about the vent. Also what does the rest of the room look like and is this the best location for a TV?

Yes, only wall possible.

Can you take a picture from table looking toward the TV?

Center the tv and lovely cabinet on that wall and then reassess to determine what else would be needed. However, I don't see a built-in for that area.

View from table

Oh wow that is a very large sectional. I agree no built ins. Get a longer table for the tv that may have shelving or closed storage and fill the space with that but a built
In won’t be great here.

Your home has lovely views and is nice and open. But there is a scale issue that makes little sense - large entry, huge open space, vast windows, large seating area, ooops architect forgot wall space for a TV!

So the TV wall looks smooshed and adding a fireplace is going to make everything on that wall feel even tinier and more cramped compared to the vastness of the room.

Instead, try to make this area feel wider and more vast. Hang the TV centered between the left wall and the start of the stair case. Hang it a smidge lower. Get a shelf that runs the entire length of that wall. Add a few decorative items to the shelf. That's it.

If you need storage in this space, get console cabinets for behind the sofa, in between the two columns and then to the right of the columns.

Sorry, no room for a decent looking fireplace in this space. Watch a fire on TV instead!

https://modernblaze.com/collections/electric-fireplaces

Why prpane ? Just do an electric FP so simple the ones that give off great heat are best if you require the heating part .Certainly a lot less than 6K. That space is odd . I think Kendrahs idea is as good as it gets if thta TV has to stay there . You can get a wall hung linear electric FP that would look a lot like that ledge she shows .

Some inspiration here

or

Our of context, this looks great. In a small home, small living space. But in your actual space, that is so large and vast, I think it would read as a dinky afterthought.

Also, what do you need and intend to put on built-in shelving? Shelves don't make sense to me as a decorative statement.

You obviously have at least one child. Do you need storage? I doubt you need shelving really. I would get a long storage unit I could put toys and games in and then either put tv on it or mount on wall. I would get a tirade ottoman CS coffee table ajd I would find discreet storage options throughout the space too as they will be needed.

Crying in my SINK! The answer to every ill in design is not a built in!

Got 64" next to what you have? This yours? Get more!

Save your 4k - 6k of total waste, and use it for seating that suits your space. No to a contrived faux fire, that would never be there anyway.

Get a match cabinet to what you have. Looks like it will fit! Move the tv to right- NO need to center on the cabinetry.

Call your HVAC guy, and have him install a TOE DUCTOR. Beyond simple, done alllll the time! . It forces the warm air right out of that vent to front and up.. Stop obsessing about it.

Clear all the junk you're parking next to and below: )

If you're stuck with a huge sectional butted to the supports?

Get a couple matched OPEN consoles. tuck big baskets below for the inevitable kiddie clutter of toys. You've not ONE lamp in sight......: (

Draw and measure the entire space. Or show the floor plan with the supports. Might there be even better ways to arrange furniture? Yes.....but nobody can say without the feet and inches, and location of the posts and more context .