Tristan Taylor has less cards than Dartz and less relevance as a duelist, and they added him.
Same goes for Lumis & Umbra. Your argument is INVALID, every time you say something like: They won't add this X character, your argument becomes immediately invalid. Damn, you guys need to learn your lesson.
Meanwhile, Dartz's theme is "Orichalchos". That's very specific, and unless they start throwing us a bunch of anime only cards at once, Dartz is unlikely to be in DL.
...It follows every canon, it doesn't stick to any one in particular. Also, even if you argue the DM is largely focused on the manga (even though Pegasus is clearly alive), the GX portion's main focus is its anime, where Pegasus is alive, and thus can't be canon to the DM manga.
It's also worth pointing out that some of the character dialogue only makes sense in context of the anime. Tristan's dialogue for Acrobat Monkey for example.
The storyline follows the manga for the DM part (since not many read GX, 5Ds or the others), with some bits from the anime. Inglorious Bastion uses his manga deck with Gozuki and Mezuki, for example.
As for Dartz, aside from The Seal of Orichalcos, only Divine Serpent Geh remains as his ace card. But seeing that we already have Rafael's cards (same with Gecko and Sherry)... Not a chance.