You could put shelving into the smaller alcove and paint the inside a different colour from the rest of the wall. You could also purchase real brick tiles that look far better than paper. You could glitter paint inside the alcove to contrast against the brick.

是的 如楼上说所 做一个颜色的区分 使得中间壁炉部分的墙体独立开来

I'd paint the entire wall a darker colour and place the TV in the niche. Either on a shelf or on a cabinet if you can find one that fits or can have one made. It will draw your eyes to the fireplace and take away the focus from the asymmetrical wall.

I like Carolina's idea to put TV in the existing alcove. You could also get a tradesman to make the other side of the fireplace look similar and add shelves there (and mirror or artwork above fireplace). I don't think it matters if one alcove is wider than the other. We have that in our sitting room and it looks fine.

The finish on the 'new' alcove will look good if it is plastered and painted (to match other side). Or you could ask builder to use mdf/similar to fill out the top and then wallpaper the chimney breast and the the bits above the two alcoves to match. Not sure - just an idea.

painting or wallpapering the back of the alcoves would be interesting. It seems that that room could carry a dark colour, but it doesn't necessarily need to be a dark colour. It seems to me that light paint colours can work well as accent walls, and wallpapers have lots of ways of adding interest - texture, stripes, patterns, geometric shapes - without needing heavy colour.