As shown on the official anime site, as well as in the teaser trailer, "Saiyuki Reload: ZEROIN" will adapt the second to last manga arc of "Saiyuki Reload." Known as "Even a Worm," the arc is well-regarded among fans (via Crunchyroll). With "Even a Worm" as the focal plot point for the upcoming anime, this will essentially mean a reintroduction of antagonists Hazel Grouse and Gatti "Gat" Nenehawk.
Part of the arc already received the anime adaptation treatment thanks to "Saiyuki Reload Gunlock." However, that adaptation made some significant changes from the manga, such as the backstories and introductions of antagonists Hazel Grouse and Gati "Gat" Nenehawk and their concluding battle with the Sanzo Ikkou. "Saiyuki Reload Gunlock" only used a few key story points and ideas from the manga for its adaptation of "Even a Worm." The anime deviated greatly from its manga source (via Silicon Era). But with "Saiyuki Reload: ZEROIN," the anime could aim to stick closer to the manga this time, perhaps giving this arc a "Full Metal Alchemist: Brotherhood" treatment. That would be a real crowd-pleaser.
