This sounds pretty good so far, I like a lot of things! Let me run through and give my thoughts.
This sounds good, building up to a final climactic showdown between the brothers. Maybe also include a scene where the Toa have doubts about awakening Ekimu as they "remember" the devastation the brothers caused when they fought the first time. Maybe they get overconfident and think they can take Makuta on themselves? Heck, maybe it can be a character arc for someone, maybe Tahu.
I see where this is going, but I'm not too keen on the Rahkshi returning, at least as primary villains again.
I like the keystone idea, reminds me of '08, but I don't know about another fetch quest. It also makes the Rahkshi problem worse as it makes the entire year basically a retread of Year 1.
I realize the Toa do have to accomplish their goal, but having them win every single time kind of defeats the purpose a little. There's no point in plotting out these encounters if the Toa never face a setback. Again, it also makes the Rahkshi even less important.
I like this, this is a good threat for Makuta to pull, especially since his beef was with sentient life in the first place. If the other years are done right, this should also be pretty emotional if we've had the chance to really connect with the island and its people.
Yes! Callbacks to Metru-Nui are always a plus in my book!
Okay, but having the Toa only succeed in awakening Ekimu after Makuta has been defeated causes a few problems.
First off, the balanced sides and the climactic battle between brothers the premise promised us doesn't happen, which is disappointing.
Second, it makes awakening Ekimu in the first place a moot point. If the Toa were able to defeat Makuta by themselves all along, then why do they need to go after Ekimu?
Lastly, and less importantly, it makes Ekimu himself, at least at this point, nothing but a MacGuffin and an exposition dumper when he reveals Karzahni.
I like this idea a whole lot better. It solves the retread issue by giving the Toa a new threat to defeat while hopefully leaving the Rahkshi out.
I don't think the sides really need to be balanced. And besides, six powerful Toa against one massive, very powerful army and its commander? It's not even in numbers, but it's still a good challenge for the Toa to face, and they haven't yet faced a cunning commander and a massive army yet.
I can see Makuta, being without the Brotherhood, making a deal with Sidorak to unleash the Skull Spider army and keep the Toa busy as he collects the Keystones and goes to destroy Ekimu himself, thus ensuring he can't be stopped.
When the Toa defeat the hoard and interrogate Sidorak, they discover the spiders were just a distraction and that Makuta is already about to destroy the only being capable of stopping him forever.
Now from there the Toa can fight Makuta, and maybe he revives the Rahkshi for a final hurrah (I'd still rather they be left out). Now, the Toa could probably Toa Seal Makuta away, but if they do this, something should happen to the Keystones that prevents Ekimu from being awakened. The Toa can always seal Makuta, but only Ekimu can truly defeat him, so if the Toa only succeed in stopping him temporarily, it will create a lingering threat that Makuta will come back and Ekimu won't be able to be awakened to stop him.
The other option is that the Toa get their butts handed to them by Makuta and something happens that causes Ekimu to awaken then and there and stop Makuta. Maybe, to keep Makuta around, Ekimu is weak from his eons of sleep, so he only manages to stun Makuta, allowing the Toa to seal him away. Then we can have a de-powered Ekimu join the Toa, all of them hoping Ekimu can regain his power in time before Makuta's eventual escape.
You may even combine the two. Maybe Ekimu only awakens for a very short time, long enough to help the Toa seal him away. The Keystones are destroyed by Makuta just before he is sealed, and Ekimu falls back into slumber, leaving the Toa worried about Makuta's eventual return and if Ekimu can ever be awakened again.
Meanwhile, a new threat looms as the third brother, stirred by the clashing of the other two, begins to wake...
Anyways, I do really like this pitch, I just think you should go with the Sidorak alternative. Really excited to see where it goes!