Potentially anything, If you look through the SDK alone you will see some of the abilities it has. With the time and know how you can modify anything sent and received form the server to create a "cheat" such as end game cash, exp, god mode, etc.
I've done this also and I've created some pretty odd things that never made it to the public like the "gm" look while in game. Turning your player into a wireframe model, and even just dropping a pile of gun's( it was pretty damn funny ).
But the point is: If it's controlled by the server and ever reaches your player you can change it.