Wargames run the gamut from simple to complex. There are some things that AxA doesn’t attempt to model in a complex way, such as logistics and physical transport, weather, random events, and so forth. These are all simplified into the movement and money system. I think it strikes a good balance between the simple and the complex, personally.
Then there are some things, such as new unit types, new national objectives, varied production facilities and technologies that are essentially add-ons to the game. These can add fun, but also add some complexity. The full unit pallette seems to work pretty well in my opinion, without many gaps. However, AAA, Cruisers and Tacticals are rarely purchased and push the variety that can be supported in a d6 system to its limit–the unit choices in the Global game seem about all that can be done without making the units compete with one another for the same roles. Conversely, bombers seem a bit overpowered and transports subject to varied treatments. Overall, it feels as though with some units being less efficient, adding more can only really add to the imbalance of the pieces against one another.
Then, we also have extensive rules to change how the game is won, how setup works, and overarching rule changes (such as single blockers cannot stop armies or every team have different unit costs/types). These address balance, but are likely to create new imbalances.
This is a mature game, and so many people are fairly bored with the OOB setup, rich though somewhat imbalanced as it is. This leads people to homebrew rules, event decks, game aides, special unit molds, and a huge variety of rules that cover everything that all wargames do (varied resources, politics, historical events like winter war and spanish civil war). As a result, we could list almost 10 vetted and tested ways you can play (just!) the Global game once you add in the spinoffs of global scope (G40, G40 BM, G41, G42, Sired’s Rules, GW36/39, YG VC rules, YG Deluxe Edition, The War Game), and similar games (Ikusa, Fortress America, War Room, and on and on). That goes even deeper on Tripple A, as people have made d6 “AxA Clones” for dozens of historical and non-historical scenarios, not all based on world wars.
The essence of this is just like DOTA is a modification of Warcraft 3 (1999), and games like Civ and company of Heroes, Cities Skylines etc. have their own Steam based homebrew communities creating new content, so does AxA. In the last 10 years, the movement has gone from one of trying to monetize small changes, to one of allowing the community to mod the game freely, developer tools being shared, 3d printing! etc–you’ll find that this community is where most of the innovation is happening, without any profit margin or motive, and not at WotC. The motives of game designers and large markets are completely different from internet communities, and this has created the Second Golden Age of Gaming–games are no longer limited by mass market appeal, legal trademark ownership, licenses, printed ideas, large companies, but rather by …