“if you design a “perfect” air spring, whose positive and negative springs create a totally linear spring rate”
Impossible anyway regardless of any heating effect.
The curve of an air spring is C shaped and always well be. As you compress the piston the volume you are compressing gets smaller and smaller and therefore it becomes harder and harder to compress further.
You can “flatten” the C shape and make it “more” linear by increasing the volume of the air can but to make it completely linear would require an infinitely large air chamber…somewhat impractical.
a negative spring changes the characteristics but is only has any effect in the first 20%-30% of the stroke. Not only that, the negative spring is itself an air spring and therefore also a curve. So what you get is an S shape.
but as has been mentioned…. linear != Best …at least in my opinion.
Otherwise why does it feel better adding volume spacers left, right and centre?….Making our curves even curvier than they need to be.