Well you got the hole there, you might as well use it for something. I would add a safe in the hole and cover it with a picture.

Can't see the rest of your space, but I would have put the TV there and floated the sofa in the center of the space.

What is under the staircase? If that is just an empty space under there, surely you can figure out something to do with it. A hidden door? Storage shelves? Pulll out drawers? If it's accessed from another side, then yes, I would fill in that hole so you can do something functional with the wall space.

that would be a cool spot to do something like this (if you can move the couch)

extra storage, a reading nook, pet bed area, desk area, book cases,,etc. if you can and want to, open up that hole and expose what you can.

if you don't need any of this stuff, then drywall over it and say sayonara. you could finish off the wall similar to this, and redo the stairs

If you can open the entire underside of the staircase and create storage there, whether accessible from this room or whatever room is to the right of it, do -- but hide storage space behind doors. Otherwise,

Wouldn't waste even a small storage space. Remove that existing white trim and trim around the entire inside edge of the opening. Then cover your hidden storage with art or mirror in a wood frame with hinges and a magnet latch to hold it closed.

Either open it all up like Beth has shown or just cover it up completely. It just looks odd n useless.

That is where I would have put the TV at proper viewing height with that niche closed up.

Take off the molding and hang a picture. Or just cover it up with drywall.

we had a triangular one. we walled it up, it felt so good having it gone. we hung art in the space.

I’d close it up, or at least hide with a picture

It's meant for art display, and that's how I'd use it. Then again, I collect Native American pottery and basketry, the size and shape of which lends itself nicely to a cubby such as that. If you have nothing you'd like to display there, then removing the ledge and covering it with drywall to create a smooth expanse would be your best bet.