I believe, and have some evidence to support this, that Hero Factory is in fact a sequel to Bionicle. Now, I first formulated this theory when I saw this guy
Akiyama Makuro, the guy who founded Hero Factory. Now, look at his design. It looks familiar in a way, doesn't it? The feet in particular have a very.... familiar design. in fact, one that another character with cold coloration used.
Makuro is Turaga Takanuva. Over the years the Matoran and the Agori forgot about the Toa and Makuta and even became fully mechannical. But Takanuva remembered, augmenting himself with technology to keep the memory of his long dead brothers and sisters alive, eventually creating the Hero Factory to create new "Toa." To power the Hero Cores, he gave up his Toa power to turn them into Toa Stones, infusing each hero with the values and ideals of a Toa. Even his name, Makuro, is a corruption of his original name, Takua, which he can only barely remember. He also repurposed the Toa canisters into Hero pods that could travel the stars. However, his power was not the only one that powered a Hero Core. For so many years not even he could remember, Makuta hid in whatever body he could find, until he entered the hero core of Von Ness, corrupting him into a new servant.
One last explanation is that the Hero Factory building is the Mata Nui robot's foot.