Maybe you could add a shop that has "older" guns for sale? Things like a Famas and an AA-12 could be added. They'd obviously be a bit weak, but might have firing modes or some interesting feature with them.
For example, the AA-12 is an automatic shotgun that has little recoil. Maybe have the gun to be weak (2 damage a bullet, no stacking damage), but it has little recoil and and is a fully automatic gun with a large drum magazine. That's obviously not balanced, but just a thought.
Thoughts?