In my case things are different. Though my project will be closed-source, people will still be able to modify it through the editor that will be available for download. In fact, people will be able to build their own world setting with it. Of course the editor will not offer the same flexibility of changing code directly but it will let people add, change and remove pretty much all of its core functionalily. This includes attributes, formulas and a shit load of many other things.
The editor also adds one major advantage over an open-source project without an editor, it allows everyone to change a game without needing coding skills, which I believe that will capture the attention of more people. But hey! I've been wrong before.
In a distant future and if everything goes well, I plan even to let the user to edit the game engine itself so you can change, remove and even add your own functions into it.