Sibling categories instead of parent child?

No need for a whole new system. You could do this by extending tagger:

  • Every thread is required to have one (and only one) of these 6 tags.
  • Create a simple filter for tags, similar to the “All Categories” drop-down. Use this to show/hide the tags of your choice.

With more advanced filtering controls, you could even do queries like:

  • Show only categories: DOTA2, LOL
  • Show only tags: tournaments, events

And you would get a stream of posts exclusively about those two games, with either of those two tags.