My favourite out of your ideas is Helryx. It would be interesting to see her take on a more traditional Toa role, up to and including self-sacrifice.
I’m not sure about killing off Tahu and Kopaka, though. I feel like, had the story continued, Kopaka would have become a more individual character, finally striking off on his own and going on solo adventures with Pohatu.
Tahu would certainly be an interesting choice, though it would depend on the circumstances. I think he was too important and powerful of a character to kill off just to show Marendar’s power, but maybe he could die much later in the story, once Marendar has already been established and the new story arc was more developed.
Takanuva’s kind of in the same boat, especially since we know he was going to be an instrumental character in the upcoming war. But again, if it was later in the story, after he’s already done whatever he does with the war, then maybe.
My personal choice out of the Toa Nuva would be Onua. Probably while saving Lewa.
There’s a few characters that stand out as having extraordinary power that Marendar likely isn’t resistant to.
First is Tahu, with his Makuta powers. As far as we know, Marendar is still vulnerable to those powers.
My personal favourite choice, though, is Tuyet. We know she has the Toa Power of hundreds of Toa, which would likely provide some sort of advantage. The real reason I lean towards Tuyet being important, though, is for story reasons. I could see Tuyet defeating Marendar fairly early in the story (not necessarily kill, but defeat), and then using the recognition and adoration to start building this universe’s version of the Toa Empire, creating a large group of loyal Toa with her surplus of Toa Power.
Ultimately, though, my final answer to this question is that I don’t want to see Marendar defeated. Not permanently, at least. It would have been cool to see Marendar as an actual character, questioning its pre-programmed duty, just as many Toa and Matoran did throughout the story. This would also create a neat parallel between Tuyet and Marendar. Perhaps Marendar would even be the one to take down Tuyet when the time comes.
Just out of curiosity, why did you link Tuyet with the Federation Of Fear? They never actually interacted in story.
The Faxon only works on Rahi, and it can only gain their powers (not abilities).
(Unless maybe the Great Beings combined Rahi-making techniques with whatever else they used to make Marendar?)