Any Ideas on the Next "Big Thing" that Will Bring the Watchtower Down?
Dreamer. Nothing will "take the Watchtower down" unless they changed some or all of their core doctrines to be the same as orthodox Christianity, eg, they adopted the trinity doctrine, as happened with Herbert Armstrong's former religion after he died. That would be the only thing that would cause such a mass stumbling it would cause the downfall of the Watchtower religion as we know it.
I found this post on JWS, and speculation there says it's some kind of financial improprieties about which the Watchtower is writing a letter.