This is pretty much complicated so let me summarize what you are up to and then will move towards my suggestion.
You have 2 domains at the moment, one targets US and the other target AU market. Now you are planning a global launch on a third domain where most probably you are going to redirect your existing pages.
The global domain is not with you so you want to go with a temporary strategy and that is to create a sub folder. US sub folder on AU website and AU sub-folder on a US website.
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WOW, saying that this strategy is ideal for one or not will be difficult as there are lot of things missing which includes brand reputation, market share, audience trust on the brand and more.
I mean if you can wait for few months than going for a new domain is a better idea than going for a temporary strategy as this will be a very quick change for audience, first in November and another change in January.
Try to give the amount of time to audience in which they can adjust. Again, if the audience’s trust level with the brand is good and they can accept this kind of quick change then I don’t see a problem.
I am assuming that on sub folders you will link to another website instead of creating a new content on a sub folder. I believe create separate content on a sub folder is not a good idea as SEO issues and ranking will take months and within few months you have to move to another strategy.
I don’t see a problem with moving to a new domain but you have to keep in mind that when shifting two different website to a 3<sup>rd</sup> domain you will see lot of ranking fluctuation so if you have a good migration plan but no back up plan then you should consider having one right now or else you will see a big loss from the organic traffic.
Again, there are lot of points that can be discussed but this is what I think it is having the limited knowledge about the domains and business.
Hope this helps!