A while back, I had a thought for who could narrate G3's prophesy opening theme. After seeing the cast consider Norik for a bit and settle on Ikir, I've decided to actually post my original thought just for fun.
The narrator is Karzahni.
There's a method to my madness, so bear with me.
The legend tells of the brothers and of the start of the Toa to fight those brothers. So who better to tell that than someone who was there? Yes, Ikir was there too, so having him say it the very fist time to Narmoto in episode 2 is good. After that moment, each following episode starts using the legend in the theme. Using a different voice to narrate those ones isn't too strange.
Before you bring it up, yes, Karzahni was already imprisoned at that time of the Toa stopping the brothers. However, he has his Mask of Alternate Futures to show him those moments, so he would be aware of how the legend plays out too.
So... An "unknown" voice is narrating the openings after Ikir gives the first official telling. The audience hears it for each episode over the 3 years. Finally the Toa meet Karzahni. Here's how I see it playing out.
The Toa enter the area where Karzahni is hiding. As they walk in, we hear the narrator speak for the first time outside of the opening theme. He tells a sort of continuation of the legend that references the current Toa and their journey over the 3 years. The screen then pans over to Karzahni as the narrator is finishing. We see Karzahni actually moving his lips as the final lines are said. The narrator was him all along.
Yes... what we all thought was just a random voice speaking to the audience during the theme was actually just Karzahni talking to himself. Is it really that surprising that a being who has gone crazy from visions of multiple futures is spending his time talking to himself? He sees this specific legend play out in his head more than the others, so he started talking out loud to give this version more of a voice. And in the end, it was the version that came true.
Prior to thinking of Karzahni, I had imagined Voriki narrating with the same sort of reveal when they meet her. However, with Karzahni, you can have the whole "crazy talking to himself" explanation as an additional bonus.