Shinobu and his gang of seven, like the capital sins of humans...
Shinobu Sensui and his demon partner Itsuki kind of look like the Sanada & Yukimura of Yu Yu Hakusho, don't they? Although they are basically what Yusuke and Kurama would have become in a few years had they coupled together and turned bad. I think that is also why they are so prominently together in a good part of this arc. Hiei is either MIA or sleeping most of the time, and at some point Kuwabara will become the "damsel in distress".
In this arc, people of a certain area start developing superpowers that resemble those of the nen users in HunterxHunter, and some of them are clearly the predecessors of the Genei Ryodan. The basic difference is that they need to bring the enemies within their own personal "territory" for the powers to work. To explain that and test the mental preparation of the main quartet, we get rivals-soon-to-be-allies like Shadow, Taboo, Copy and Tapping.
But the real enemy is the Sensui gang of "seven" : Dark Angel (Sensui), Gatekeeper (Itsuki), Doctor, Sniper, Seaman, Gourmet and Game master. In fact, that number is not what it seems, but that's for later. Their combined goal is to destroy the barriers that keep the demon world and human world apart, and let the demons kill all humans.