People live in new house usually has less concerns about FengShui than people lives in older houses. Just like a person, younger people have lesser injuries than the elderly. In an old house, the previous owner might have a add an extra beam to a certain wall to prevent it from falling down. Some day we might have to put in new stairs to the basement. We might even have to build an elevator in side the house because grandma and her wheel chair is moving in with us. Renovation of the house also leads to temporary issues. The half-done new fire place has two protruding corners in the living room. The to be finished walk-in closet makes the bed room look like missing a corner. 
The best advice to such issue is either to start (or finish) a home improvement project. Extend one-side of the corner to reach the adjacent wall to make the 'missing' feeling disappear. I am sure your friends and family will have plenty of ideas regarding exactly how to execute this project.

There are, certain time, we can not make any change to the room itself. Here are some ideas you can try. 

  • Trees are our best friends. Put a rubber tree or anything with big leaves at the edge of the protruding wall or close to where the missing corner would be. If you have naughty pets that enjoy killing plants, hang a spider plant from the ceiling. Some believes a fake one will do the same trick, I personally do not recommend it.
  • Many people like to put a high power lamp by the supposing missing corner. This is good if you do not have the patient of taking care of plants. The best kind are the so called tree-lamp. It has multiple light bulbs on one lamp pole. This type of lamp allows you to adjust the individual watts of the bulb and their directions, which makes it quite practical.
  • If you ever paid attention to the mirrors in the dressing room, you know how great those tri-fold mirrors are.  Get a bi-fold mirror and place it at the corner. You can even buy two of the cheap mirrors and hinge them together so the mirrors can stand by themselves. This one is strongly recommended if the missing corner is on one of your good sectors.