How come Ryou doesn't have her name listed in the opening when it show's all of Tomoya's female love interests (besides Fuko)
Wasn't it because they were showing the major(ish) characters and the routes? Kyou was more of a major character, and Ryou only had her OVA, not much of a route.
Probably because they had to make some cuts, and in the series, Ryou is not a very important or included character. I do wish that they had shown us more of her personality. I really liked her.
I think female character listed in Clannad opening only show all of major arc that being told in the story, not all of Tomoya's female love interest.
Instead Fuko, in fact Kotomi doesn't show a love interest to Tomoya too.
About Ryou, I think there is a help from her sister, Kyou to confess her love. So that is reasonable if we include it in Kyou's major arc, although in After Story : Kyou's Chapter seems a litlle condradiction in that problem.