If you make another game (just not "PlaneShift") with the sourcecode, you can make your own world and races too. Then you will also need your own gameserver (and it will not be a "PlaneShift" game server, but a "your game" server).
Although the "source code" is OpenSource, the whole game "PlaneShift" content is licensed, and its content has to be approved by Luca "Talad" Pancallo. You cannot add anything to "PlaneShift" as you like -- then it would not be "PlaneShift" anymore.
You may change the source code to make your own game out of it, if required. But you should publish the changes to the public domain too. And those changes may probably not enhance "PlaneShift", but possibly only "your game"... If you want to contribute to "PlaneSHift" specifically, you must be part of the team, and cooperate with the hierarchy of the existing development team.