The filler goes on, but where is Zaraki and Yachiru? I understand they’re not gonna be on this filler because Tite Kubo probably hasn’t come up with a sword for them (Best bet is still Gashadokuro), that doesn’t mean they can’t make a reasonable excuse. Take Yamamoto for example, at the beginning of the filler they (The bad zanpakuto) clearly state they sealed him because his sword Ryujin Jakka was hard to convince, now that makes sense (I doubt they really could seal him, but what the heck). Though I bet since they already bothered to design all the other zanpakuto and their human forms he could be making an appearance at the end.
Another fact is Yoruichi, where is her sword? does she even have one? Is she even a Shinigami for all that matters? I mean, sure she was leader of the stealth squad but she was never a Captain of one of the 13 squads She was, that doesn’t mean she’s a shinigami though. As for the other captains, the manga has already shown most of their abilities, but we now all of the captains except Zaraki have bankais so we’ve still to see Ukitake’s and Kyoraku’s.