User blog comment:B214/Question and Answers 6/@comment-36.72.199.46-20161213181208/@comment-9595671-20161214091311

I agree they may not even show these two Longinus in the series, but that doesn't mean we won't ever see them. Remember Ishibumi has plans for a sequel series focusing on Millicas.

Since some Longinus were given more focus on the Slash/Dog story, it makes sense that if some don't get properly explained or showcased in DxD then they'll probably show up in there.

In fact, I'd like to think that since Tobio and Issei, both the main characters in their respective parts of the franchise, have a Longinus... then perhaps the main heroine of the sequel series will have one of the remaining Longinus to continue that pattern.

Actually, Ladon has no association to Yggdrasil because he's not from Norse myth, he's associated to greek mythology as a tree and it's guardian serpent that Hercules visited as one of his many labors. Nidhoggr is the dragon that's related to the tree, and only because he eats parts of it.