Stereo panning shouldn't be hard to add. I have programmed a few synthesizers and generally I include a panning, volume and stereo separation knob.
Linear panning is simple. Use a float value between 0 (left) and 2 (right), with 1 equal to the center point. When the sound is fully left, the resulting left channel is Left + Right. The resulting right channel is 0. Vice versa for the right channel.
With linear panning as the sound moves from side to center it seems to move closer then further away again, peaking in volume/percieved closeness at about 45 degrees.
Circular panning gives a more realistic impression of a sound source moving around your head. Rather than a linear relationship between pan position and volume / mix of left and right channels, the relationship is nonlinear. You can use any parametric function to define the relationship but generally an inverse square law ensures the power of the sound remains at the same percieved level as a sound pans from left to right.
Various stereo seperation techniques can be used to give a wider perception of the soundfield than your speakers or headphones can really produce, thanks to some interesting psychoacoustic effects. A basic technique is to sightly delay the left or right channel by a few milliseconds. By adjusting the delay you can give an interesting panning effect without changing the power of either channel.
You can make a stereo field greater than 100% panning. If you invert the left channel and mix it in with the right channel and vice versa, you begin widening the percieved stereo field. The more inverted signal you mix in, the greater the percieved surround effect. Combine this with channel delay and circular panning, and you have 3 basic tools for creating more immersive sounds.
Then we could always add HRTF's...
