Sorry but to me this is a fairly ludicrous notion. The practicalities would prevent it alone!
Where to draw the line? The original post didn't include New Zealand, someone else said what about Russia. To try and find a basis to unify all EU states, the US, Canada and Australia would require such a broad set of criteria that Russia would have to meet them - unless it was based solely on economic wealth. And then what about South Africa? Chile? Argentina? Where do you draw the line? There are similarities between all of them!
Would each state retain its own protectionist trade measures? If the US did not drop tariffs and subsidies on wheat and lamb, and Australia did not stop protecting Salmon farmers from Canadian imports, and the EU did not drop tariffs that keep out many New Zealand dairy products, etc then most of the benefits economically would disappear dwon the drain.
What about integrating legal systems? Sorry but when I look at the US legal system all I can say is please don't let Australia end up like that. Which authority of law would prevail on a particular issue when they are different? To keep it geographically different would negate the effects of unification. And does this legal system use a British-orginating common law system, a Roman-Dutch system, a French Civil system?
What about setting up a federation as was mentioned: do each of the Australian states, each of the Canadian provinces, each of the US states each become a member of the federation? Then that would be hugely unfair on New Zealand, the UK, France, etc which are all unitary. Tasmania (or Delaware) would have the same power in a multi-member constituted house of parliament as the whole of France!
And a system of overall government: do Australia, New Zealand and the UK have to accept a Presidential style of central government? Or will the US and France accept a parliamentary executive?
The differences are so great that overcoming them would come close to bankrupting the whole country. Look at the financial cost of reunifying the BRD and DDR: it is still costing all Germans a fortune, and the social division it has created is enormous. Imagine trying to unify France and Germany into one country!
While the idea of simplified trade and travel appeal, as do wamr and fuzzy notions of global unity, national pride is still too strong to allow it - and in my estimation will remain so. The EU is our best guide. Depite WWI, WWII, and the Cold War Europe cannot even manage partial integration - the reason: national consiousness.
The way to achieve the benefits that would flow from unifacction into one state are through reduced trade barriers, free trade, unification of both private and public law (through UNIDROIT, the ILC, ILO, ICC, IMO, and ISO), the implementation of the Rome Convention's Internaional Criminal Court together with consent by states such as the USA to the jurisdiction of the International Court of Justice.
International law and free trade are the tools to achieve unity and economic growth.