I have two related suggestions for increasing the variety of units for Allied Corps as well as the early defensive campaigns for the Allied and Soviet Corps. I'll lay it out for the Allied Corps but the same principle applies to the Soviet Corps.
One, while you can build up your core within the DLC to what's needed for gameplay, but much like GLC 42\43, you can only bring a small number of those units from the previous DLCs into the next DLC. Then you rebuild your core up again, and perhaps again. Sure, some gamers won't like it but it will keep things historically balanced.
Two, I think it's fair that to improve gameplay, the programmers will look past the strategic imbalance and focus on actual battlefield numbers. They'll do this may making many of the scenarios small-scale so that even a strategically outnumbered side will have local numerical superiority. 3 Infantry Divisions can then easily turn into 24 Battalions with associated artillery, anti-tank and support units and should be fun for most gamers, even if their own core forces morph from division to battalion scaled units from scenario to scenario!
Based on these two ideas, here were my thoughts for the early Allied DLCs if they are ever developed. I'll lay out my thoughts below, jump on me if you like.
First, I believe that Allied Corps was lacking the "allied"; you were either British or US, and you had aux minor nationalities. Although the extra air power was nice I would have liked to see the Allied SE or bonus units be Polish, NZ, CDN, FF ground units each with their own special characteristics. Just some thoughts were Polish and Greek Mtn troops, Cdn paratroopers, the Free French Infantry with extra mobility, Indian Heavy Weapons, etc. Of course, they should be upgradeable into '43 stats. Then you get the extra ground power plus some of the multinational "realism". Perhaps SE or perhaps something along the lines of inserting German special units in the v 1.14 update.
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Two, for the early Allied DC I think it's important to not look at the strategic imbalance or even historic inevitability by focusing and narrowing the scale of the battle into some fun and gameable scenarios without getting too far offtrack.
Building on my first suggestion, your first units are
Polish. As we all know the Polish defensive campaign was full of small scale counter attacks and successful defenses that the DC could focus on. This defensive campaign can result in a climatic Battle of Warsaw, an ahistorical breakout and a fairly historical retreat to the Rumanian Bridgehead. While there should be a fair-sized core during this DC, you are allowed to bring only 2-3 highly experienced Polish units into the 1940 DC
that you cannot buy again until 1944 unless they pop up as SE. This is historical, btw for those unaware of the 1st and 2nd Polish Divisions that were part of the French strategic reserve in May 1940.
For the 1940 DC, you have your small Polish core force imported and are given enough prestige to restock with a
French core for this DC. Again, many opportunities for successful defensive scenarios and small-scale counter attacks aplenty here to get your gaming on and to build experience for core forces. A successful defence in the Dunkirk area could be followed by either an evac off the beach followed by a redeployment in unoccupied France (which occurred for the French forces) or an ahistorical breakout through German lines back into unoccupied France. This could be again followed by defensive victories over Panzer spearheads and a final evac out of France, and an opportunity to bring over 5 elite Polish and French core units, again with the inability to buy these units types again until 1944ish or so unless they come in as SE.
For DCFall 1940\1941 your small but highly experienced core can finally be filled out with British and ANZAC forces with scenarios ranging from the Western Desert to Greece, Ethiopia, Vichy Syria or parallel tracks dealing with the German invasion of the Balkans or in North Africa. It'd be a good touch at this point to bring in a Greek Mtn SE\bonus unit. If you brought French or Polish tank or air types, the UK tech tree would be the obvious upgrade path until at least 1943. After the Fall 1940\1941 DLC, you would carry your full core over into future DLCs.
I actually very much enjoyed the Allied Corps scenarios in the Western Desert, and I am not sure where and how those would dovetail together, but you would then have a fully multinational core force, and you can watch your Polish 3TP from Poznan develop into a Comet in Berlin.
You'll notice my thoughts involved two ahistorical breakouts from Warsaw and Dunkirk, but I am comfortable with it, and I know the programmers are as they had the ahistorical breakouts from Stalingrad and East Prussia, and I've never seen any criticism about them (nor should they recieve any, those were great scenarios).
And for any Soviet Corps DLCs the same two themes should apply. Either reduce the scale of the battlefield for playability and\or a limited core import between the first one or two DLCs to keep things balanced, or maybe an ahistorical breakout or two for fun.
I'm not a modder and although I'm interested in the mods, I'm not sure enough of myself to start monkeying around with files to try them out. Just wanted to get some thoughts out but I always know the next DLC gives me my PzC fix for new content, and eagerly await the release of more.