Heres a thought, how about the UK and hence the British Commonwealth decide to be neutral in the lead up to WWII.
My thinking is the UK was a preeminent power because of her geography and her navy, they could choose to avoid continental wars most of the time and following the cost of WWI decided to do so again.
There is no doubt they would eventually be dragged into the war as even if Hitler was inclined to leave Britain be, Mussolini likely would have done something stupid on the assumption the Germans had his back, i.e. attack Malta or Egypt. There is also Japans aspirations in South East Asia.
Assuming the UK is neutral for the first year or three of the war, did not suffer the early defeats and loss of manpower and equipment and were able as such to develop and introduce new equipment, tactics and strategies in light of what they had seen happen across the channel, where would they be when they did get involved. More to the point what would be the trigger to end their neutrality?
On the modelling side we obviously have vehicles, aircraft and ships that were killed off as there was insufficient time to introduce them, platforms that had to be used as is that instead could have been improved and technology that existed but in the panic of a shooting war it just wasn't possible to introduce.
For example the UK was well aware of the advantages of sloped armour on ships and tanks, in fact specified it on the Crusader and Covenanter, but dropped it for the A24 and following designs. It is quite conceivable that given more time the Crusader, let alone the Cromwell, could have been a properly designed Medium Tank, superior to the T-34, and in service by Britain's entry into the war.
Then there are the many aircraft and aircraft engine designs that could have done with more time Westland Whirlwind for example, maybe the DH Flamingo, I'm sure everyone has their favorites. There's the extra time the FAA has to get the aircraft they need as opposed to the ones the RAF said they needed, time for the Army to convince the Air Board that they definitely need something different to the Lysander, i.e. AOPs and dive / fighter bombers.
The RN could complete extra rebuilds of capital ships, the rest of the QEs, Repulse and Hood all rebuilt to the same or similar standard to QE, Valiant and Renown. THe KGVs could have been completed with 16" guns as the US was able to do with their North Carolina Class and the Lions could have been designed completely free from treaty restrictions. No treaties also means instead of having to esign their carriers to a weight they could have had both Ark Royals size and Illustrious' protection, i.e. the Armoured Fleet Carriers could have started at closer to the size of the Audicious Class.
All this new and upgraded gear would have seen the availability of surplus equipment for transfer to the Commonwealth, in particular Australia and New Zealand to better counter Japan and South Africa to address the Italian threat. A lot of tanks, artillery, vehicles and aircraft to South Africa and obviously ships and aircraft to Australasia. Dominion industries would also have been bolstered to become more self sufficient but also to reinforce Mother.
I wonder if perhaps it would be possible for the Commonwealth to remain neutral even after Japan attacks the US and only enter the war once it become apparent that Japan is going to take Malaya as well as the Philopines and Dutch East Indies?