Rooms with funky surfaces like that can be a blessing and a curse. They're very difficult to predict - not to mention ones like the pic above would tuck the speakers into a really tight place which is not good.
If it was me, I'd look for a large room square or not. If you're serious about really good sound, you'll be building another set of walls inside for isolation anyway and you can change the dimensions at that point.
Room volume, room height, symmetry, ease of isolation, access to HVAC, ability to cover windows, etc. Those are the things I'd look for. I'll start with a 24x24x10 space any day and tweak it to what I want - like maybe a 10x19x23 room with a nice storage closet, a couple of nice deep bass absorbers, good spacing between inner and outer walls, etc. Think about finally having a GOOD place to store all your boxes and crap - in that 3.5'x24' closet on one side of the room!
ALL rooms suffer from modal issues. There are no perfect ratios - just those that try to spread them out instead of having them stack up on each other. You'll still have to deal with them and the decay times, reflections, SBIR, etc.
You can also shorten the room a bit and then recess a couple of nice 2-3' deep, 2' wide, floor to ceiling bass absorbers. Now that will reach down into the bottom end and not take up any of the visible part of the room. The space between can again be a closet, equipment rack, bathroom, etc.
Oh, and don't forget a built in kegorator and tapper in the room.
Bryan