I'm loving this.
Just one idea: Toa Ignika returns. The Ignika's mind somehow gets free from Mata Nui's and is able to create a separate body for itself. Since Ignika needs a lot of energy to keep both his and Mata Nui's body existing, Ignika can't access none of it's powers to help survive/defend themselves from the experiment. This would cause a harder challenge for the team.
Thoughts?