Cedric Diggory comes to mind, especially since he plays an integral part in book four, and is a main driving force behind Harry’s grief in a good part of the fifth. Sure, Cedric was briefly introduced in book three during the Quidditch match between Hufflepuff and Gryffindor. Given how important he proved to be, I believe that he deserves to be mentioned more in the script for S3 {if the plans continue as stated where each book gets made into a season}.

Him being introduced in the show through the match would be wonderful, and perhaps the match could be one of the focuses so that Cedric’s exceptional Quidditch skills are translated from pages to screen. The writers can also add him to other scenes, as long as there are valid reasons for him to be tain them. I wouldn't want it to be a case of 'oh, there's Cedric' while we're watching the match, then he makes an appearance until S4 having been forgotten after Quidditch is completed. That way, the fans can potentially form a bond with the character and when he’s sadly killed off, his death will be that more profound. As bad as that sounds from a Cedric-lover’s POV, his personality and death will be remembered if done right.