Cabernet is the girl with manly eyebrows that shall show up in BW019.
Well... Ash has a rival in Shooti (haha his name is Trip) sorta, and kind of a friendly rivalry with Bel. Iris doesn't have any kind of discernible ambition so far, which kind of excludes her from getting a rival like the two girls before her have had. So if anyone were to get a rival it would be Dent, which would change things up.
I suppose Brock never had a rival because... how would you show that? Like... a rivalry in Pokémon breeding would be rather boring, because it's all about patience, hard work, superb care, and Pokémon-raising skills. They wouldn't want to get in-depth with that in a more action-y show, I don't think.
Er... I just kind of went off there... MY TRAIN OF THOUGHT MAKES SENSE TO ME, DON'T QUESTION IT