Mask of Light: Rewrite the Matoran relationships, and focus less on them and more on the Toa. Get rid of the stupid Kolhii match at the beginning. Make the Rahkshi and Makuta more threatening. Actually have Takanuva and Makuta duke it out. Get rid of all the inexplicable stuff that makes no sense, like Takanuva turning the giant Mata Nui head into a giant Takua head, freezing Turahk gold, Lewa using Tahu's sword to create fire, Lewa carrying Pewku, lack of mask/elemental power usage by the Toa, and the entire ending. All the voices need to be changed. Dialog needs to be rewritten. More exposition for people who aren't familiar with Bionicle. Characters need to be redesigned.
Legends of Metru Nui: Focus more on Whenua, Nuju, and Onewa. Totally change Matau. Don't give Matau two mask powers. Make Makuta more threatening. Once again, new dialog, voices, animation, etc. Take out all the cringe-worthy slapstick humor.
Web of Shadows: Fix all the glaring plot holes, so probably a complete script rewrite. Focus more on the Toa besides Vakama. Have Vakama turn evil more slowly, and have the other Toa also kind of go crazy. Totally redo the Rahaga. New voices for everybody except possibly Roodaka, but give her voice actress some acting lessons or something, because her performance was kind of obtuse. Take out any attempt at humor.
The Legend Reborn: Nothing in that movie really works. Everything's got to go and be completely redone.