I play this game about 4 times a year with friends. We’ve tweaked the rules are few times and now we have a pretty much optimal setup. The only added unit are trucks (basically land transports for Inf, Art and Tanks) and it makes for interesting moves and plays.
We’re of the mentality that historical accuracy should not get in the way of replayability and fun, so rules such as limiting the number of units in India, or changing the US involvement makes for more and more linear gameplays that are basically the same strategies happening with the dice becoming the sole factor in the game. We like to try new things.
Also, adding new units in a d6 game where the unit costs are single digits makes it almost impossible to balance correctly. Every time we tried tweaking land units values and costs we basically destroyed a unit and made another one completely OP. I think you would have to switch to a d8 system and multiply costs by 5 (and income of course) to allow proper tweaking. Also, it’s very easy to make units that sound good on paper but end up being fairly situational.
We’re dabbling with the idea of allowing players to build one extra factory at the start of the game, which would make for some cool tweaks. I am also thinking about bombers that would be expensive units that deal damage automatically and you can’t block it via tanks (though again, it may be situational). We’re also thinking of making air combat optional or last just one round as well, with the winner being the one with the most air planes in the end. We’ve had a few games where it was a 7v5 air battle and it makes no sense to lose so much IPCs in one go in a game that is about grinding, especially when this means you basically can never again challenge the enemy’s 4 hit artilleries.
All in all, the game as it is is pretty much fun and can become even better with few adjustments. The trick is to add an element that has good synergy with the current game meta. Otherwise you might as well just make a whole new game with its own rules.