There are a number of interesting things about Rats such as their need to chew (as their teeth continue to grow). They are constantly gnawing on things and can chew through a wide variety of things such as bone, wood, cinder blocks, lead pipes (not steel plating though), etc. Maybe you could have a Bite ability as a melee attack, but also as a way to dig through certain terrain/obsticals? There's also their keen sense of smell, which can so sensative as to detect Landmines and even diagnose tuberculosis. They can also purportedly carry and transmit a number of diseases, such as bubonic plague, lassa fever, leptospirosis, Hantavirus, etc.
If your going for some anthropomorphising, you could also try mining some other works for idea's, such as The Secret of Nim, The Redwall Novels, The Tale of Despereaux, the rats in Dishonored, etc.