I’m not suggesting that the game continues until the Russian civil war is fought to a finish, merely that the conditions post revolution are possible. The RCW started in 1917, and involved the Allies and Central Powers.
http://en.wikipedia.org/wiki/Russian_civil_war
So…
The Russian revolution occurs. We assume that Germany sends Lenin in to take control.
Russian units are now split into two factions: Bolsheviks (reds) controlled by Germany/CP, and Mensheviks (whites) controlled by the Allies. Perhaps allegiances are determined randomly; more realistically those units still on the front line are more likely to remain loyal, those surrounding Moscow to be solidly Bolshevik.
Germany gets full control of the Bolsheviks, but these units cannot operate outside Russia proper, that is essentially those areas within the modern Russian Federation. G/CP must evacuate these areas, but can move into and take possession of any other European Russian starting areas (those listed above by IL plus Finland). In reality the CP set up several puppet states, but they were firmly under CP control and supplied vast resources to their armies.
White Russian forces remain in play and controlled by the Allies, they are free to move and attack CPs and Bolsheviks anywhere.
If Reds or Whites control Victory cities (assuming these are part of the game) they still count towards winning conditions for either side.
Russian ships are subject to the same conditions; Reds can remain in any SZ adjacent to Russia proper.
New units can be built by either side under appropriate conditions (i.e keep separate incomes for each).
Since the CP will be free to throw its weight into the battles on the Western Front, the Allies will have to decide if its worth keeping the Eastern War going by sending forces into Russia, so forcing the CP to keep units there.
http://en.wikipedia.org/wiki/Allied_Intervention_in_the_Russian_Civil_War
A few more suggestions:
Only the CP are allowed to attack neutrals. As far as I know, the Allies didn’t do this in WWI, with the dubious exception of Persia, which was already partly occupied.