Received the pm poitead.
I would fisrt address the concrete floor. Acid washed or not, that will pose a huge problem. A thick carpet and pad will go a long way. Wall to wall would be best, but a rug and pad leaving a foot or two about the perimeter will be a close second.
As the Archway encompasses the entire width of the room (less two feet on either side), simply putting in a door is not a viable option. (I don't think bpape got the memo). But I don't see a huge acoustical problem with the acoustics of that openness with proper placement and treatment. And the understanding that you have more of a "space" than a "room" for your theater.
After the rug and proper speaker placement, you may want to consider one or two treatments as necessary. Imho, that should only be seriously discussed after the room is completed and the system properly installed and calibrated. Finally, per my first post, it is not that expensive to use sound isolating clips with extra drywall if you want to soundproof the space. I would install the clips and extra drywall if soundproofing is a concern.
Please keep us informed.