I'm not positive what behavior you're referring to when you say "Now, when I start a new project, that Discipline comes up". I think what you're saying is that the default views that are created are Mechanical discipline. That is the result of loading the Mechanical Template by default.
You can change the default template setting in Settings -> Options... -> File Locations tab (see attached image). Or, you can just manually select the Electrical Template manually when you start a new project.
In reality though, it's not like there are some things that can be done in the Mechanical but can't be done in the Electrical template. It's really just a matter of creating new views of either discipline. There's no real limitation.
What is it that you perceive that you need in one template that's not in the others? I'm intrigued.
Kyle B
Revit MEP Product Manager
Kyle Bernhardt
Director
Building Design Strategy
Autodesk, Inc.