One change a friend of mine added to his board when we were in college was to add a sea zone off Denmark, splitting the north sea and the Baltic, and populating it with mines.
The mines functioned as “anti-aircraft” for ships and was “owned” by whoever held Berlin. (each enemy ship entering, roll dice, a 1 = a ship sunk)
Since most folks strongly agree that the game strongly favors the Allies, this little twist was actually a nice addition.
It prevented Russia from reaching the Baltic fleet in R1, made it so that UK needed twice as many transports to land in Karelia, Eastern, and Germany in order to land the same numbers of troops each round (with the risk of losing ships in and out of the Baltic).
It was JUST enough to balance out the game in the early stages (first 10 rounds), which is when most games are decided.
Personally, I think it is a far better answer to balancing the game than Russia Restricted, or “Axis Advantage” that is available in the Hasbro version of A&A.