Traditionally, fireplaces occupy the center space in a wall and is lined with a mantel. However, these days fireplaces are also placed on walls that an angle. These look very unique and classy as it gives a different look to the entire room. Corner fireplaces are also more convenient to clean and decorate.
Essentially this concept involves picking out three items which are linked together by a common thread, but not identical. They could be linked by color, texture or theme; for instance three picture frames, candle sconces, or pieces of china.
Divide your mantel mentally into half and place these on one side of the dividing line, but closer to the center, ensuring that they aren't perched at the very edge. Don't line them up symmetrically, but experiment, pulling them back and forth, perhaps 'layering' them by tucking one a little behind the other two.
It may take a while to figure this out, but with some experimentation, you're quite likely to find something that works well. The principle functions on what the eye perceives as balance which is not always related to perfect symmetry.
Add a few knick-knacks or pieces around the main focal point, maybe a rosemary topiary or some souvenirs from a holiday and you're good to go. If the wall behind is textured, you could even get a simple frame, or a number of frames in different shapes, and place them against the wall.
You can create a cozy nook around the fireplace by placing a few floor cushions, a loveseat or an armchair around it. If the space allows, you can extend recessed shelves above the fireplace, or even add a bookcase, and you have yourself a warm little library.