Off the top of my head?
An Advanced Safety System with the Electro Shocker option; because The Shocker!

A bayonet! Nah, just kidding

The Gecko Grip modification; because having your 30k nuyen rifle shot or swiped out of your hands would suck...
The Improved Range Finder system; expensive, but a persistent range reduction of one that doesn't require a take aim action? Sign me up!
A Safe Target System; because you don't want to railgun your friends by accident...
A sling! For the same reason as the Gecko Grip, but spec-ops stylee!
Tracker. Just because it would suck to have this piece of tech stolen...
Underbarrel chainsaw! Because come on

Underbarrel flamethrower! BECAUSE COME ON!

Seriously, though, the chainsaw and flamethrower are interesting options, but the grenade launcher is probably a better option since it uses the same skill as the rifle itself.
Oh, and protip;
Not all critters in Chicago are heavily armored
