So we haven't sorted out how to model convoy at sea yet.
But these two should be easy and less controversial.
(When Russian cities when surrounded by German forces, all they have and can raise is infantry.)
When an inland territory is completed surrounded by hostile territories it may not transport IPC outside nor spend IPC from outside. Unspent IPCs are forfeited.
2. Interruption of production
(Fighting were not done at US homeland in WWII. US production goes uninterrupted.)
Collect one less IPC from each of your territories attacked by enemies since your last turn.
Do we need this russian isolation rule? I don't think so, because when they are surrounded they should buy mostly inf anyway. the purchasing costs already take care of this, we don't need an explicit rule here IMO.
What about territories surrounded by SZs and territories? I think this "surrounded by hostiles" idea could be a good start but needs to be fine tuned.
What's with this uninterrupted US production? How is the game affected with/without fighting on US soil?
I don't like automatically subtracting 1 no matter the size of the attacking force. Attacker could possibly exploit this. This could be a good start though.