The mods usually will look through the forums of the games that they moderate, and (most of the time) they'll consider the suggestion(s). Now, they won't always add the suggested category to the game, but they usually will look through and at least consider it. 
If you don't get a response from the mods within a couple weeks, try contacting them via their social media (usually Twitter and/or Twitch is the best).

In addition to this, using default settings, anyone who has followed the game will get a notification when a new thread is posted there.