With four kids ages (9-15), I think it's important for the master to be on the same side as the kids' bedrooms. Maybe not accessible in quite the same way but certainly not disconnected by being way across the house. Fires, robberies, anything could happen.

Either find a plan already designed that way or take what you have to an architect and explain what you love about the plan. I'm sure they can draft something with the same architectural features that you love but with better use of space and the # of bedrooms you desire.

Adding on a bedroom without knowing details is not really something for a forum. In most modern ranch plans, you see the master on one side of the kitchen/living and other beds on the other side. That is a huge change to this plan. Otherwise you will have to add a hallway between 2 bedrooms to get it over on that side.

You could shift all the living space to the right and give up the giant pantry, in order to add a bedroom next to the master bedroom. Not a minor change. Don't know if it would work with the elevation. You need to look for 3-bedroom plans or find an architect to design one for you. :)

One suggestion would be to add the bedroom next to the existing bedroom in front of the master closets. You could put a hallway where the second bedroom closet is to access the new bedroom. This will make the existing bedroom a little smaller though, but you could always bump it forward. If you want windows in the master closet, you could put them on the other wall.

Before going too far do you have septic? If so does it's design and local laws allow for adding a third bedroom?