This interesting SMG design by KRISS USA gets the cute treatment. It would be an unlocked weapon, once a soldier specializes.
It's an idea I'm still toying around with. I may base it on weapon experience: Use the pistol long enough, and the new class will unlock.
Another path would be to lock at a set of requirements a solider would have to perform, such as a certain number of kills, hit percentage, or damage taken.