A better solution is add a nation grind for all campaigns, and lock determinate equipment for be used from some campaigns
Like
Fg42 usable in normandy and berlin but not in moscow, Tunisia, stalingrad
And because we need unlock squad i suggest to leave the campaigns grind for only squad
So we split the grind in equipment and squad and grind both at the same time
It rise another problem:
Right now, except for flammerII radioII and engineerII, there is no purpose to use anything else than the level I squads.
But seperating the weapon grind may be the best clue toward a solution.
We can imagine not unlocking weapons, but unlocking tiers.
For exemple, weapon already got tier right now.
It goes from 3 
to 6 .
Let’s say you want tier 1 smg.
You grind the tech tree and unlock it.
In each campaign it could be something different.
Berlin T1 smg: mp38
Moscow T1 smg: mp34
Tunisia T1 smg: bereta
Then you want better smg, you have to unlock the tier2.
It’s a solution but it will be messy to understand as a new player because it’s already messy to explain.
“I habe unlock the mp38 in Berlin but it’s not available in this other campaign. BUG REPORT!!! GIVE BACK MY MONEY”
And another problem is that some campaign don’t have t3.
Right now the system is very simple:
In tunisa, lvl8 you got engineer squad with THIS weapon.
Simple. Don’t have to think.
problem is the year of the weapon. some 3 star weapons range from start of the century, towards 1943-1944 (e.g. m3). so cause of the year of the weapon (m3 in this example) you could only use it normandy, but it is 3 star weapon. and thompson that is 6 stars was available through entire war.
that is why it is better to have unlocks by year. you want to play tunis? well you must not select anything made from second half of 1943 onward. and allow obsolete equipment to enter later campaigns for memes 
hmmm maybe we should have a something like a 15%-20% bonus exp if we had that weapon in a difference exp in an ideal situation
Bump. Devs need to see this
Introduce new maps/cosmetics/ and premium squads for theaters. If you buy them they are introduced into rotation. If not, players will not see them.