Oh my, a climbing wall might be too short for the space. Do you have a door / opening between the two spaces ? You might be over-thinking this wall, perhaps just couple of nice pictures or art work will do.

We have a large wall in our living room that has a vaulted ceiling. We tried hanging our largest print and it looked like a postage stamp. We ended up buying a 5' x 8' rug in a modernistic design, and hanging it from a curtain rod screwed into studs 12' up. It looks great and always invites positive comment from guests.

Paint it with magnetic paint. All kids of possibilites open up. Or mount an interesting geometric shelf as an eye-catcher.

I would stone the center and place living walls on both sides.

Mouldings, to break up the plain blank slate and to tie the room together. Then you could start an art collection to fill that great wall. I wish I had a wall like that! Or you could have a mural painted to totally change the atmosphere.

I'd say glass block, that way you won't be loosing any light. You may want glass on either side and a doorway in the center.

Something like this http://houzz.com/photos/18474

I am including 4 examples of climbing walls because you did not specify your brother's age or what other use the room might have, such as living. I hope you will find them helpful and send an after photo when you are done! It will be a really fun room; I am excited for you! Please have the fireplace moved by a professional with the help of an engineer so you do not risk fire in the future. How about giving me some hints for what you enjoy and how you see yourself using your side of the room.

the wall will be completely closed off so no gaps to the far window. it is nearly two stories tall as is 4.30m (nearly 14') so not quite a rock climbing wall but pretty good for in house use. we already moved the fire place. for now there wont be a fireplace on either side. maybe later on i would want a fireplace like the one below. it could transition into a TV stand



i rather like color schemes like this one but i don't know how it will go with all the wood floors and windows. and besides i think i want something a bit more modern.



i was thinkın of seating like these. they seem solid and uncluttered. sadly no carpets because we are allergic to dust mites.



i also like this as a space filler for the wall where the fireplace is right now