This might sound weird.

What I mean is, if I am trying to make a new category, where would be the best place to submit it to the site?

You don't submit it to the site, you discuss it with the moderators and the community of the game you're running. They are the ones that decide what categories will be on their LBs. Before doing that, you should make sure you do a run of the proposed category because having a run to go along with your idea will be more substantial than just posing the idea without a run.