How much workout equipment do you have? Would it fit in the 12.5' bump-out at the top of your plan?That area would be easy to close off from the rest of the space to keep the gym atmosphere corralled. You could put storage under the stairs and install a system of shelves/shallow closets on the 27.2' area at the foot of the stairs for easy, quick access from upstairs. If you have young kids, that is also where I'd put the game/play area, so you can easily come down and check on them. Then past the bathroom, I'd put the theater room. If you need more storage than under stairs and cabinets, use the left end of the large space for that. Wall off the area just past the workout room and HVAC for a large storage space for sports gear and other big seasonal item storage that you don't need quick and often access to. If you have more workout equipment than the 12.5' area can contain, revers the two spaces and put the gym in the left end of the large area. A second option, if you entertain guests a lot, is to put the theater room at the foot of the stairs, so people don't have to wander through the children's area to get to the theater room. Then divide the larger space into game/play area and storage, with the HVAC system in its own small but accessible sound insulated room in both cases so it doesn't ruin the movie experience.

Thanks for the ideas. I think the workout equipment will fit in that area. I like the idea about the closet/shelves along the wall at the bottom of the stairs.

Love to see pix of what you do. Happy to give more input as the project takes shape. If you do make a large storage area in the end of the big space, you may want to install double doors so you can put big items there. Would be nice if you had a walkout and could take outdoor items inside for storage.