How do you select a good niche for your community?

[Sinistra]

So I have a concept of a community I want to start but I am trying to figure out how you select a niche that will overpower the rest. so say you have a website (geek-nexus) shameless plug? anywho I want it to be more of a community for general geekdom but I am torn between gaming, tech Star Trek, and Star Wars. as the main niche. But I want to cover everything else. Do you choose what niche you want to be a main or do you just go with it?