Canada is doable through a variety of methods, and there is a good method for earlier American settlement and shipping to the Pacific North West, but getting settlers around to Australia/New Zealand would be difficult since America doesn't have South Africa or India to stop at. Including Australia and New Zealand before 1900 is going to be more adventurous, and I think most of those scenarios would preclude the United States. Maybe the English Civil War has a different outcome.
The us never has the revolution and Britain doesn't go to india. Then Britain federalized their empire. However, with the American pop so large, the imperial parliament becomes dominated by amerivdv.
Does this work?
That's certainly plausible, though would people still think of themselves as "Americans" in this scenario? It's unlikely the BNA colonies would be federalised as Canada was IOTL, so you probably wouldn't have any government over the individual states except for the Imperial government. But yes, the Empire probably would be dominated by people from OTL's America in this circumstance.
When I meant Federalized, I was thinking more of the Imperial Federation concept.
I think the earliest reasonable cut would be Nicaragua in the 1850s. Such a project seems like an easy way to kill a bunch of people with tropical diseases and to go into ruinous debt. Maybe this would be doable if Spain never went Hapsburg, and reinvested a lot of funds into its colonial ventures.An earlier Panama Canal might help, by making it easier to get from the US centre of gravity in the east over to Australasia.