roguelike or roguelikes?
shooter or shooters?
I was just about to ask this.
From a related meta.SO question, my favorite comment was "Common sense, please. Keywords should not be pluralised. Concepts probably should. And certain words need to be left alone because pluralising them would change the meaning."
I'm going to post some suggested guidelines, one-at-a-time -- so people can upvote only the concepts they agree with.
For Genres, It Depends.
Rule of thumb: If you append "games" to the plural form of a genre, then make it singular.