In fact, a lot of things !
Depending of your time, talent, programming knowledge, you will be able to make small modifications like parameters tuning up to add new multiplayer modes for example.
Technically speaking, the following kind of data can be modified :
- NDF files : contain script and objects descriptions
- PY files (Python) : contain algorithm not easy to express in NDF
- TGA : bitmap files used in the HUD
- OGG : music files
- WMV : video for the cut-scenes in the solo missions