I know it sounds like something out of a Star Trek film, but the Imperial Federation truly was an existing plan. Proposed in the later half of the 19th century, the plan was to incorporate the British colonies, including the largely self governing ones like Canada and Australia, into Britain itself.
The proposal was to create a single federal state among all colonies of the British Empire. The federation would have a common parliament and would be governed as a superstate. Thus, Imperial unity could be maintained while still allowing for democratic government. The colonies would increase their influence while Britain would be able to share the costs of imperial defence. The best features of large states could be combined with the best features of small states.
It was seen as a method of solving the Home Rule problem in Ireland, as England, Scotland, Wales, and Ireland (along with the other members of the Old Commonwealth) would have their own Parliaments.
There were two main obstacles of this plan, the first being the vast distances between this lands. Though Edward Ellis Morris pointed out that it was now as easy to reach London from Melbourne as it had been to reach London from Orkney at the time of the Acts of Union 1707, or to reach Washington, D.C. from California before passes over the Rockies were made.
Another obstacle to the scheme was what one of its proponents, Richard Jebb, called colonial nationalism. The granting of authority to a super-parliament composed of many competing interests was seen by opponents as a compromise to the powers of the local parliaments. Leading colonial supporters of imperial federation, such as Australian prime minister Alfred Deakin, however saw the movement as a way to increase the influence of the dominions over imperial defense and foreign policy. The colonial branches of the Imperial Federation League in fact outlived the demise of the home branch in London, which collapsed in 1896 when it failed to resolve internal disputes over imperial trade policy.
Then World War 1 occurred and all hopes for the movements died out with it. What is the smallest change that can be made in history to allow for the Imperial Federation to exist?