I begin by repeating myself: if adding units, add more territories. Given the current map (G40, 2nd edition), I honestly don’t see any room for additional units. More space would need to added to open up additional possibilities for strategic movement on the board. This would create a new problems of its own, which I address below, but it is the fundamental change to which I think one must commit when deciding to add more units.
So, the real question is: How to balance realism/plausibility with a desire on the part of some players to do “new stuff” with new mechanics. Several major challenges arise:
Are we looking at the operational or strategic level of warfare? Axis & Allies splits the difference for the sake of creating a reasonable fun gameplay experience. If each unit is an abstraction of actual forces, then:
Land units (mostly) represent armies or corps of infantry, and divisions of armor and artillery. A few odd units in places like Hong Kong and French Indo-China represent functionally independent divisional commands. I have heard some concern on these forums about artillery moving and attacking at the same time as other units. Some people think that artillery should fire first, as it does during a general advance on the battlefield. There have also been arguments that it should fire in defense. I think that the first argument is most-flawed. Because of the level of abstraction described above (typically at the corps and divisional levels), land combat in Axis & Allies represents strategic, campaign-level movement in a given region. It does not represent operational or tactical activity. Artillery should not be presented separately or be able to conduct discrete “fire missions.” In terms of using artillery on the defensive, I think the choice to do otherwise was dictated by use of the d6 standard. Infantry don’t gain a defense bonus from artillery because they would defend at 3, which would dramatically reduce the value of purchasing armor.
Air units usually represent full wings of fighters or bombers, but the Scramble rule is all about using them tactically. The chief problem with air units has to do with movement. They perform regular movement at one scale, and combat movement at another.
Naval units represent flotillas. Thus, each destroyer represents a group, and each capital ship, a squadron. Together, they form fleets. My only gripe about naval action is that the Cruiser and the Battleship are not sufficiently different. Let’s compare.
So what about the map?
My recommendation would be to increase the number of map spaces at the expense of realism. Each turn still represents about three months’ real time. Crossing the Atlantic would take 2 turns. Crossing the Pacific would take 3-4 turns. This would not be realistic, but it would create more opportunities for movement, blocking, and naval combat searches, which I think should be instituted. (I think that, before any naval combat, a die should be rolled to determine whether the searching fleets even find one another. This creates the possibility that attacks on enemy fleets will be foiled and tempts players into trying to “sneak” forces past the enemy.)
Air units should have different values for attacking other air units and then naval and land forces. Split the current tactical bomber into two units: a naval torpedo bomber that can select which naval units are hit and a dive bomber that is slightly cheaper, but optimized to hit land units.
Does the Cruiser have good value?
The value of a unit can be calculated by adding up its movement, attack, and defense values, expressed relative to its cost. Thus, a cruiser has value 8/12, or 2/3. That is, you get a .66 return on every IPC spent. A battleship has value 10/20, or 1/2, which is a .5 return on every IPC spent. However, it can survive an extra hit, which means that it is technically a 1 return on every IPC spent. The obvious cruiser alternative is more destroyers. A destroyer has value 6/8, or 3/4, meaning that there is a .75 return on every IPC spent before factoring in the benefit of its anti-submarine capabilities. Better, then, to buy destroyers or battleships every time, especially because one can buy 2 destroyers for every 1.5 cruisers. (I’m no math whiz, and I welcome alternative interpretations.)
One approach to improving the cruiser’s value would be, I think, to give it a small area-of-effect bonus against attacking aircraft, representing its role as an escort for the big battlewagons.
In terms of adding other units, I think Commerce Raiders would be useful only if more spaces were added to the game. The inclusion of trade routes in the new HBG 1936 edition may create space for such a unit. I like the idea of adding Destroyer Escorts to safeguard convoy zones and transports, but I think that kind of work can already be performed by the Destroyer. Torpedo boats would be an interesting addition, but their value is really in the Mediterranean, which would need to be expanded in space to enhance the value of maneuver. I’m also concerned that such small craft wouldn’t thrive against A&A air units, which are extraordinarily powerful against other units.
Battlecruisers are interesting but effectively pointless since they will not travel at 3. HBG has experimented with Coastal Battleships and Coastal Submarines for lesser powers, and I suppose these units might be worthwhile, but then only as part of the forces that smaller nations receive at game start and are compelled to use due to anemic budgets.