@Daz:
It was a minor detail adding to the background of Bone Guy, it wasn't a tease of things to come;
To be fair neither was the revelation that the final villain(s) were Uchihas and neither one being Sasuke to some people.
Everyone just assumed Orochimaru & His sound Ninjas and then The Akatsuki were going to be the final villain(s) and then got swerved (repeatedly).
@Daz:
Kaguyas appearance in the end is still executed as a clunkyly inserted asspul… unless you're saying that 1 offhand mention of a bone-fighter clan by Orochimaru 400 chapters earlier had you wating years in anticipation of the moment where Kaguya would finally claim her rightful place as series Big Bad.
Well considering Pain leapfrogged Orochimaru, who then got leapfrogged by Obito, who then got leapfrogged by Madara for the position of Big Bad it'd be a tad bit unfair to single Kaguya out for doing it too. But yeah you can't really defend Kaguya's sudden appearance and promotion to bigger bad in arc which at the point was running for too long.
@Horizon:
It was a blatant asspull. All you'd have to do is give her the name and abilities of a clan and call it a day. It's very simple.
Sasuke cheating death in his fight with Hidan was an asspull.
Killer B somehow surviving having Gyuki removed from his body was an asspull
The Sage Of The Six Paths Transcending death and then giving Naruto & Sasuke a powerup in the middle of the fight with Madara was an asspull.
Kaguya showing up out of the blue and wreaking havoc seemed little more than Kishimoto just wanting to throw another villian into a story that was already bloated and past it's sell by date for some people.