Seriously, she at least has more than 3 cards printed, and she has a generic "girl" deck so Konami can throw any "girl" card for her drops/level-up rewards.
Dartz literally has only 3 of his cards printed (Shunoros, Seal, and Geh), and none of them works with each other. And his deck theme means you can't just slap random cards as his drops/level-up rewards.
Seal, Geh and Shunoros are all better cards than any of the cards Serenity used, however.
I don't see what's stopping them from doing Dartz if they really want to. They could go for a snake theme or something if they really wanted him to have a theme, or else just go for cards based on ancient stuff. It's not like cards like Twin-Bow Centaur were heavily Orichalcos-inspired anyway.
Oh boi Kaiba technically owned dartz in and the pink rose hair guy in the series. Since they are adding non manga characters such as Sartorius, Hassleberry and Chumley I don't really see any reason not to give us Dartz since plenty of people remember him.