This is really exciting. My friends and I have been speculating and the global event could, we think, possibly be the release of Mew Two. They now have the raid system up and this is the perfect opportunity. Just organised an impromptu trip to Chester with my girlfriend and our uni friend purely because a Pokemon go event is happening there.
Our theory is this:
Phase 1:
The event I am going to go to, as UK timezone is ahead of America is a prequel event to Chicago's Grant Park event. Chester will temporarily have 120 pokestops throughout the city. That is more than London and it is much smaller. That is perhaps a poke stop for every 20 steps, more than in any city in the world apparently. ALL will also have lures. Imaging the amount of Pokemon appearing. Phase one needs to globally achieve a certain percentage of people maxing out certain medals. Chester is the perfect place for this and also Chicago will be too.
Phase 2:
This is Chicago's job. They need to complete this to unlock the last reward for the world. Three gyms will appear with raids on. One for each team. This makes it seem like the raids will be the legendary birds. If the three raids are all successful that means all people attending the Chicago event will get one of the legendary birds.
Phase 3:
Phase 3, if Phase 2 was successful will see all gyms become raids for two days. The Pokemon being Mew/Mewtwo. This gives everyone in the world the chance to get them and possibly if they succeed, the raid will return but with the other so people can get both.

Of course this is a theory but it seems to fit perfectly. The coincidence of the Chicago and Chester events happening at the same time and also that raid are now a thing, the legendaries can have a legitimate way of being caught. What other time than the anniversary would they do a mass legendary release?