My take on that would be to try it for the next global zday event. It’s always a surprise where these events take place. A bit more transparency would be great.
It would also push chapters to already think ahead about a plan. Because with the co-initiative, you would describe the basic setup of the ZDay you may offer. Location, ticket fee, seats, recording/stream capabilities, potential speakers in mind and such. Being able to host a ZDay fully in English is a must in my opinion as well when you opt for a global event. Live translation takes the fun out of it for me. But yeah, that’s me 
With that information we can compare this with other chapters. Then all chapters may of course continue their ZDay idea, but we’ll select one as the global event based on how well prepared they are and what they can offer
Having a global event in the spotlights is great for publicity. We then as one global movement promote a single event, which then takes it up a notch compared to previous ZDays (in theory). So it might also act as an inspiration for other chapters to do better. And might also bring up the question in the general public; “is there a ZDay/chapter in my country as well?”
And having the planning transparent might also help other chapters to organize their events, because they then have a template/runbook for such an event.