I think we need historical incentives so that things are likely to play out the way they did, but certainly not necessarily:
USSR gets +5 IPC a round while on the Axis side. This is an incentive for Germany/Italy to attack (plus somewhat realistic, as Hitler invaded knowing that if he waited, USSR would only be more powerful).
If Germany invades the USSR before the USSR invades Germany, the USSR is unable to attack German forces on its next turn.
When combat takes place in the United Kingdom or American forces are attacked, America joins the war.
Japan has a victory point win condition similar to pacific. However, Japan does not obtain victory points until America has entered the war.
The first time Japan attacks America, all American naval units defend at 1. However, America receives a Purchase Units and Place Units phase before combat is resolved (to prevent Japan from walking into western USA).
All USSR infantry and artillery defend at 3 against Japanese units in red territory.
The game ends when a power fulfills victory conditions. That power wins. All conquest-based conditions require the power to retain control of the area conquered for a full round. Victory conditions are as follows (2 per power):
Germany/Italy:
Victory if Moscow is under German/Italian control.
Victory if the United Kingdom is under German/Italian control.
Japan:
Victory if 24 victory points are reached.
Victory if Hawaii, Australia, and India are all under Japanese control.
Axis USSR:
Victory if the United Kingdom is under USSR control.
Allied USSR:
Victory if Berlin is under USSR control.
USSR:
Victory if Russia retains all original red territories, plus at least 15 IPC of other territory.
Western Allies:
Victory if Berlin, Paris, Rome, and Moscow are under Allied control.
Victory if Tokyo is under Allied control or Japan fails to collect victory points on its turn.
Tie:
If USSR and the Western Allies each fulfill their victory condition at the same time, whichever side has more IPC income is considered to have the advantage in the cold war, and therefore is the ultimate victor.