There probably won't be one big stumbling block, but rather numerous little ones that eventually have a cumulative effect. Having said that, going forward, likely the largest stumbling block that will present itself will be when the system blows in a way that Jehovah's Witnesses have not anticipated. That will be a very difficult thing for the Watchtower to deal with, particularly since the financial system will at some point blow out. That will present a scenario whereby it may not be possible for the Watchtower to even carry on their publishing operations in the way that they have been accustomed up to now. Jesus of course foretold that before the end would come that many would be stumbled and that many would betray one another and hate one another.
Obviously that development from within God's organization is well under way and will only accelerate and intensify further as we get closer to the end. It was of course in that very context that Jesus said that the one that endured to the end would be saved. Apostate Witnesses are performing an essential Judas-like fuction that fulfills numerous prophetic Psalms. / You Know