I hate to say something can’t be done. You might be able to do it, but it would probably add too much complexity for what it will be bringing to the game. Bridges/rivers were very important in WWII tactics, but it does seem very hard to introduce in the game.
Here’s an idea (forgetting about complexity for a moment), have a list of historical “bottlenecks” during WWII. I use the general term bottlenecks to refer to any key points such as bridges, mountain passes, etc… Each bottleneck connects 2 different territories. Western Europe and Germany are connected by the Rhine, for example. At the start of the game, all bottlenecks are “open”, that is that there is no penalty for moving between the respective connected territories (in this case W. Europe to Germany and vise versa). During a player’s turn who controls at least one of those 2 territories may destroy the bottleneck (i.e. blow the bridge/ mountain pass road, etc…). Until the bottleneck is repaired (cost of 1 IPC and 1 full turn to build maybe? I don’t know, we can discuss that later.), attacking units have some penalty (such as -1 attack strength) when moving from 1 of the territories to the other (i.e. moving from W. Europe to Germany or vise versa).
Maybe other bottlenecks could include the Burma Road, the Alps (connects Southern and Western Europe), Yellow River (connects China and Manchuria), etc…
This idea needs some work but it’s not too bad for coming up with it on the fly like that, right?