Is the foundation poured already? Like BIG said, space is tight, better goes 24' long and 16' wide if you could. Also, do 9' ceiling if it is not too late. What is up with the unexcavated space there? That is a perfect space for a very nice size home theater.
As far as HVAC goes, I have to deal with mine be that relocating one main trunk from center to the side wall. So, my advice would be,
1. Don't run two main trunk together in the center of the basement, you will have a huge bulk head that you need to frame soffit to box them in
2. Use flex duct cross the theater space in joists
3. Run every duct in the joist cavity
4. Plan ahead where you want your basement (including theater room) supply and return vents go and have the HVAC guy pre-install take-off fittings so you don't end up with having to cut holes on main trunks later.
5. Install duct liner in main trunks
6. Do a dual zones if possible
7. Put the HVAC main unit and hot water boiler somewhere near the corner so you don't wast space.
Another non-HVAC but related point, all the pipes (gas, cold & hot water and waste water) should run as clean as possible, you do not want to deal a lot of obtrude pipes here and there, a lot of headache dealing with those pipes when you finish basement, framing is more challenging.