I have joists in the basement, 8' ceilings. So I was going to put 1x3
pressure treated down, shim them up, and cover with OSB for the
subfloor (this is on top of concrete, no water problems). Then carpet
over that, sound good?

Basement tends to have a higher humidity level than the rest of the
house because the HVAC system probably will not take care of the
basement. Stepping on a carpet that has a slightly higher humidity
level may have a kind of sticky feeling.

Drop ceiling... While I'd rather have drywall ceilings, drop will make
it so much easier for running pipes, wires, etc, in the future. Do you
frame around main air duct that splits the room or drop the ceiling
another 6" to cover the whole thing?

I like drop ceiling for running wiring. Many time we simply cannot
forsee what kind of wiring that we may need to run in the future. A
drop-ceiling takes care of this possibility.

I would not want to drop the ceiling by 6". You has 8-ft ceiling that
is reasonably good height. But then you need to subtract the floor
height and the drop ceiling height. You may end up getting 7-ft left.
If you need to drop the ceiling further by 6" for the air duct, you
will only have 6.5-ft left. That is a bit too low. And a low ceiling
will subtract the "value" of a finished basement (here "value" means
the value to you in term of comfort and having a good feeling, and is
not in term of dollar value).

Seem like the air duct runs along the center of the room, you can frame
around it, and kind of using it as a way to divide the room into two
areas to achieve a "rooms within a room" effect -- like one side is a
bar another side is an entertainment center. And you can put furniture
around it to re-inforce the idea.

Jay Chan