You have recorded chants, yells, and other phrases with a 50 or so odd group of people - how would you make it sound like thousands?
You have the whole stereo field to play with.
You could use any plug-in.
Theres only so much you can do by layering the same material - how would you use DSP to create a larger arena from out of a small group of people? (Other than the obvious worldizing reverb)
My attempts consisted of pitching duplicate layers to mix, shifting a left channel slightly off time with the right, using stereo wideners on specific layers, and using delays to either add more character to the reverb or to smear others into a layer of a slightly unintelligible element.