What about if attachments are unlocked as you level?
When you unlock them, you will be able to purchase them from the trader gal.
Imagine flashlight attachments, laser pointers, holosights, scopes, foregrips that will increase accuracy, grenade launchers, maybe motion sensors too, and why not some improvised attachments, like big bayonets or chainsaw-bayonets?
You will start with a few "basic" attachments (like a relatively weak flashlight for all perks and a weak holosight), and later you will unlock better ones.
Each weapon will have only some available slots, so things like the one in the example below will be avoided.
Most guns will be equipable with after-marker sights, but some guns like Lever Action and .44 magnum will not.
Guns like the AK will have a foregrip/GL slot and also a sight slot. Guns with lots of rails will have more slots.
Now, about current weapons with attachments.
These weapons will be left the same, and you will have the option to upgrade them if you wish. For example, a non-demo player will be able to get a GL for their AK, but a Demo will be able to buy the M4-203 as it is, in its current price, an they will also have bonuses in damage, accuracy and reload, and also discounts that non-Demo players will not.
Attachments will be general (flashlights, foregrips), or perk-specific (scopes and pointers for SS, holosights for commando, blades/chainsaws for berserker, GLs for demo, mini-flamers for firebug, and something for the medic too.)