Instead of all the Assault Rifle and Submachine gun weapon categories, they should make this.
You select one weapon, for example the AK-47. You can put simple attachments on it, like the GP-25 or Dual Mags. You can adapt it to other variants of the weapon, like the AKS-47, RPK or AKM. You can put different attachments on the variants.
And for example the SCAR rifle. You can adapt it to the SCAR-H (Heavy) or the SCAR-L (Light). The lighter version provides higher mobility, but less accuracy.
Infinity Ward should make a lot of weapons, so you have more weapons, and more variants to choose.