Huge amounts of noise can be generated by large drums, brass instruments, acoustic guitars, the human voice, clappers, bells, gongs, horns, a steam calliope, bellows powered organ etc
Sound is amplified by water as the waves boost the signal - Greeks used water between the stage and the audience - fantastic lighting can be achieved by using gas-fired arc lights
an old 'roman amphitheater at Triers used to seat 20,000 people - a fitting choice -or maybe a natural formation on the water somewhere that will accommodate 200,000 if you need to.
maybe get opera stars that are trained to project their voices to participate if you really need to hear the words
WHAT A GREAT DEMONSTRATION OF ENERGY CONSERVATION (of course, we'll have battery powered taping systems for broadcast)