I understand the feeling against Sheetrock. However sonically it is not much different to painted or varnished wood. It is easily controlled. Light tongue and groove varnished wood is an attractive alternative with a nicer 'feel' and IMHO a 'nice' acoustic. Slats with treatment and fabric would sound and look good. This is similar to the pegboard and other ideas mentioned. You can see this type of thing at John Sayers site.
Focus.
The final goal is a balance.
First treat the corners with most of your resources and energy. 34inch SuperChunks are good. Wood slats across look great.
Ceiling and side reflection points are next. 2 inches of treatment should be plenty. Do the chessboard thing. If you are really feeling it, I don't see any great reason not to treat the whole ceiling.
At this point listen. Handclaps will easily reveal any flutter echo. A sheet of 703 wrapped in fabric is very useful here. Clap, find a twang, get a friend to move the 703 until a spot that nukes it is found. Honks can be provoked by grunting. Go on, you know you want to :-) Vary your grunt pitch to provoke high booms and honks. Again the 703 can be moved around to find the taming spot. This may all read as quite idiotic. However, I found the claps and honks to be of similar use to Measurement and Calculation in achieving my White Room.
SoundSound - Gear
You will probably end up with quite a lot of treatment, but with placement based on observed twangs, honks etc. When they are gone you should have a room which will to make itself inaudible on your recordings. Localised panels can be placed around (and above) vocals or other sources which like extreme deadness. Localised ply panels can be placed around drums for the opposite effect. I have heard of rocks from the beach under snare drums.....
Good Luck, DD
PS:- Maybe I am even OLDER than Ethan......