A landing with a turn of the stairs, is about the height and what code will demand there. I expect you will hear that is a minimum of 7' of headroom between a landing and the header
that is supporting the sides of the stairwell.
Whichever way you turn, will require one step down from a landing if you build a landing at the second step height. I expect that would be between 14" and 16". You also will need to decide where to enter
the theater, and which way it is orientated. That landing could be elongated and allow one to enter at riser level, if so desired. There's lots of space and lots of options, so it is maybe more about how
inexpensive one could build substantial, with a new home coming in at the top end of a budget. You also need to work with those beams and ductwork, and make sure there's space for spot lights and
It appears you will need an ejector pit and pump for the waste line, and that lower area where the waste line clean out exits the house, or the front wall area in the front corner, where all
the waste lines are, would make for a logical location.
I don't if you want the room volume but I could see that space being even more open if you simply kept the stair area open beneath and let the stairs float in the overall space. Now that would be
more finished space and more cost.
You can play with ideas for now, but you might find the builder or trades, do each home a wee bit different, and the plans are basically just a rough estimate.