A simple one but can help :
For every unlock of the weapon
Your grind accelerates for other campaigns
Example :
I unlocked MP40 on Moscow
Now I unlock other MP40 levels 2x faster
Another unlock accelerates to 3x faster
Another to 4x
and so on.
Not a new idea but still a good one i think.
Could be a huge time saver for all the campaigns.
What do you think?