I have a room above the garage where I would like to place a flat-screen TV on the wall. Only downside would be that as you walk in the room you come in parallel with the TV and would be able to see all the wiring/wall mount. I would like to make and inset for the TV so that it is more or less flush with the wall. The other side of the wall is attic space, so no loss of space will occur and no need to worry about hitting electrical or plumbing (I can get to the other side of the wall no problem through the attic). My idea is to essentially frame an opening as you would for a window (header, sill, jack studs, etc). I would like to go more than 3 1/2" and create, lets say, a 6" cantilever. Do I need to reinforce that? Is a cantilever into attic space a bad idea? Do I need an architect/inspector to create a "plan?" Any suggestions and recommendations is much appreciated.

I would not cantilever this box since the weight of the TV will be carried on the back wall of the box. Since the attic space is open run studs floor to ceilng creating the back wall. I would also brace the back wall with a few studs horizontally out the back towards what I picture would be the roof line or another wall. The back wall of the box will want to bow out at the bottom

I think the idea is fine. Just a few questions.. the opening is 2x4 studs and you want to cantilever the sill using a 2x6?
if that is correct you should be fine.. the TV weight should be OK for that.
I also think you don't want to go much deeper because A) the viewing angle & B) ventilation
there are other options.. but let us know if this works