To toss in my 2 cents:
I have no problem with Afrika Korps (or other similar future efforts) being it's own stand alone dynamic campaign.
However I will toss my voice in with Zhivago's that I would like to see an additional GC41 and GC42 set in Africa serving as a bridge between GC40 and GC43-45 west. (ie this is something I wouldn't hesitate to pay for).
As for whether or not to link the GCs, or at least link them at a single point at the end of 1940:
The ideal would be able to play all of them together and switch between them at any point but the issue is it means they all have to be the same size and scale and difficulty so really limits what we can do and also doesn't fit the history very well. The kind of force you build for an Eastern front is very different to what you would have on Western front as you have very different terrain and opponents and the scale is significantly different.
In the original PG you could choose between fronts at several key points in the campaign. The big advantage of doing so is adding replayability to the DLC as a whole: being able to choose between Leningrad OR Kiev added some replayability to the DLC - being able to choose between the east or africa in 1941 then again in 1942 would increase the replayability value of the DLC *without much additional effort* on the part of the development team.
Scale in PG game, as in PzC, is ambiguous so units might approximate anything from Battalions to Divisions depending upon the scenario being played. Consequently in the context of a game (as opposed to a simulation) I see no problem having the **playing pieces** approximate Brigades/Battalions on the eastern front and Battalion/Company sized formations on the western front. The same Tiger that *approximate* a Tiger Battalion at Minsk might just as easily *approximate* a Tiger company at Goodwood. Again I repeat this is a game - as artillery only ever fires 3 hexes regardless of the map scale - I don't see the logic that units can't vary in what they represent from east to west.
Core composition is a bigger issue, however I don't think it's really insurmountable. It's really a question of balance:
That gets complicated further when each DLC is intended to be playable stand-alone or as part of the GC - players might start with a pre-set core, a core from the Russian DLCs or a core from a Allies-based GC; all would have different issues to deal with. Upshot: it's not as easy as it sounds.
Eastern front was more about epic tank battles, while in the west the focus could shift towards infantry and fighting against enemy's very strong air presense. Additional difficulties arise if we allow the player to switch between East and West every year.
Yes there are difficulties but IMO you're overlooking the fact that the developers don't necessarily have to be the ones to solve them. Perfect being the enemy of good enough:
There already ARE difficulties with the current DLC - players are given complete freedom to choose their cores. Some choices trivialise the content, some make it impossible, starting in 1945 is a lot different than importing a core that was carried through 6 previous DLCs - and yet the developers have ruled out adding additional mechanics to control the value of a players core until the next game. And yet the game still works - why? Because the players adapt - they change difficulty to suit their core, they self handicap, they cheat, they mod, they do what they need to do to enjoy the game.
Similarly to give players the option to switch fronts doesn't require perfection just:
1) Design equivalent year battles same core size **in terms of number game playing pieces**, regardless of what world scale they're supposed to represent
2) put a disclaimer that "importing an eastern force into a western campaign may not be balanced" (and vice versa)
3) leave it to the players to adapt. IF they want to switch fronts they'll have to build a balanced force that will work for both.
Lastly regardless of whether or not you link future GCs you'll never get balance 100% on -
4)
Give players in game control over difficulty on a scenario by scenario basis. ie The ability to mod difficulty levels is great but why not go the extra little step and make those options changeable within the game via sliders. Seperate out the +/- strength into several broad categories - artillery, aircraft, afvs, infantry and players can customise their difficulty on a scenario by scenario basis. If they want to play Normandy with an "east front" core then add +5 to the Shermans and -3 to the Mustangs.
Problem solved for (relatively) little work beyond creating the DLC West content and adding a few more *in game* tools for allowing players to fine tune their own balance.