Hi Patine,
Great to see you breaking ground on this one again. I have a suggestion that you may wish to consider before going too far. This is coming from a good place and from someone who has completed his share of scenarios, some quite large. It also considers some of the issues I've had to grapple with for my "upcoming" (take that with a grain of salt) Hinge of Fate, where I considered a 1936 and 1939 German start but ultimately landed on May, 1940.
I think you ought to sit down and ponder if you truly want to go with a 1937 start date. One question I'd ask you is "what is the meat of the scenario supposed to be?" If that answer is the fight against the United States, I would implore you to consider a start date closer to December 1941. A 1937 start date will pose significant challenges. Though not insurmountable, they may make this project much more than you can, or would ultimately want to, chew. Some things to consider:
-Balancing the scenario will be much more difficult. You are basically proposing to have a build-up scenario where the player presumably has 40+ turns to grow before the main opponent unlocks. Consider that Red Front was only about 48 turns in its entirety and what buildup was possible there. While you can adjust build times and shields, that's a lot of turns to leave any player to figure out how to min-max.
-Playtesting the scenario will be enormously more time-consuming as little changes that you implement available from turn 1 can dramatically affect how the early 1940s go, necessitating you to play potentially dozens of turns over and over again just to get to the part where America starts and the real meat begins. You might be able to get away with mid-point save starts for some things you change (such as events that only fire mid-game), but more often than not I'd suspect you're going to be restarting and will feel your eyes glaze over as you go through it.
-You are limiting the useful and fun units you can use by forcing yourself to use earlier units. You can minimize this issue with a batch file and rules updates but then you're creating massively more work.
-It is much less likely that you will finish the scenario, given the grand scope. This isn't simply a matter of units or rules files to consider but the very delicate maneuvers you must force the AI to adopt.
How much do you really want to see the 1937-1941 conflicts? Is this what is going to hook most players, or are people going to be sitting around waiting for the main one to start?
Might I suggest an alternative:
Use a 1941 start date but still have a "choose your own adventure" buildup where the player gets to decide how the war went from 1937 until 1941. Achieve this by using a lua event such as I'm using in Midway. There, I present the player with a dialogue box on turn one with three options. I'm using it simply to add some fog of war and allow the player to choose where their units deploy (north, south, center) so the other side isn't quite sure where the enemy is. The concept, however, could easily be expanded and you might have a series of dialogue options that describe an event from 1937 to 1941, and present the player with a few options (Occupy Indochina, Leave it Be); ("Abide by the Washington Naval Treaty; "Ignore It"). Then, depending on the decision, the starting map is different and the units are different. All of this is easily achievable in lua with scenarios that are either currently released and available or that should be released well before you're ready to start plugging away with this.
This would be much work on the front end, as it would require you to keep track of many coordinates and figure out what the ramifications of each choice would be, but it would save you considerable work on the back end with playtesting, given that you wouldn't need to replay the first 40-odd turns repeatedly just to finally get to the part you really need to test for balance.
Anyway, it's your baby--do what you want and I'll be glad to play it regardless--but I thought I'd lay some of this out there because you might not know some of the roadblocks ahead of you. If (and this is a total assumption on my part) the main reason you want 1937 is for the fun of the buildup (like in Hearts of Iron) you might achieve this via the abstract (but fun, interesting, and unique) way I laid out without losing all your hair, sanity, and sleep.