[So in the newly leaked finalized set images][1], each set has a background.
Now lemme level with everyone who’s still holding out for a continuation- It’s a reboot.
Why? Because Greg literally said they’re working on a story bible. He didn’t say NEW Story bible, but there would to be working on a story bible if it’s a continuation- because they’d already have one and all that jazz.
Anyways, as @Nyran’s said, sometimes the backgrounds haven’t had any relation to the story.
However they have had relation to the environment. Now for each set seen we have:
Tahu: A mountain in the background and possibly lava. Are they on an island made from a valcano?
Pohatu: Only a desert, we can see some ruins and Pohatu is creating/trapped in a tornado.
Gali: She’s underwater, so obviously there SHOULD be a body of water somewhere. Further support for the possibility it might be on an island.
Kopaka: More ruins, also snow.
Skull Spider: A green background, a forest possibly and also a bridge that’s been torn down
Lewa: Multiply giant pillars.
Onua is absent from this video.
So we’ve seen a Mountain in the Ta-Area, some strange area where the skull spider is as well as ruins in each area.
Also note- each set has a pedestal with a golden kanohi on it. Perhaps skull spiders are guarding these masks?
So in conclusion: My theory is that this is a reboot taking place on an island which will heavily borrow from old story while removing any elements that did not make sense later on. For example The Toa Nuva finding more Kanohi Nuva or whatever, IDK.
[1]: https://www.youtube.com/watch?v=CIsx6-MyV0w
I think it takes place in the ruins of a now destroyed Metru Nui. Maybe a little bit of a post-apocalypse vibe.
That would be radical, seeing an abandoned Metru Nui again except without the whole Hordika thing going down.
I think the 2015 story might take some elements of the Metru Nui saga with changed elements being the Protectors in place of the Vahki and the Skull Spiders replacing the Visorak. If this is the case, then my theory could be the reason LEGO’s rereleasing the BIONICLE 3 DVD.
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Wrong topic buddy…
I personally think that if they are doing a ruined Metru-Nui, they should stop and rework it. It wouldn’t make sense, esp from a reboot perspective.
If I’m right about BIONICLE 2015’s setting, then I hope they keep some facts about Metru Nui’s downfall hidden at the start of the line. It would really bring back the sense of mystery that the original storyline had.
Not sure how they could rework a core element of the storyline when the sets are a month or two away from being released… 
I know, but seriously it wouldn’t make sense to just cut straight into a ruined city with no exposition n stuff
shrug
Throwing viewers/readers straight into the thick of things is a pretty viable story-telling device (Star Wars famously did it).
I’m not explaining myself well I know.
I’m not sure quite how to put it, but it would seem wrong and weird, at least to me. Again, I don’t know why in this instance, because I love the way Star Wars did it, but oh well.
I personally think if it’s a reboot, they’ll still be in Spherus Magna, and not Mata Nui. Or maybe they’ll be in Metru Nui, except Metru Nui is a city surrounded by different biomes as opposed to being an island, more like the Lionturtle Cities that were around during the time of the first avatar from the Legend of Korra/Avatar The Last Airbender.
THANK YOU.
With that out of the way, I’d like to say that I agree with this logic. If it is a reboot (which I believe it is), it would make sense to return to a Mata Nui-esque island.
one last thing
look between the front legs on the loss set
notice how there is a cocoons
is a loss a better version of the visorak
or, is he capturing matoran/the toa
Reboot: Can be anything
Contin.: Ruins of the great spirit Robots, and… Maybe a third Great Robot?
Alright now, I would like some sort of continuation, whether it be Hard continuation or Soft reboot, but, still, I still could roll with a reboot.
But, give me a moment to grab at some straws.
The story takes place where 6 Toa arrive on a mysterious ‘island’ with no memory of their past.
They search the island, but the only thing they find are these machines who have been wandering for who knows how long.
Somehow the Toa learn what has happened to the island by searching the ruins if this once beautiful paradise.
It doesn’t take them long to find the Skull Spiders, but they know what they now must do. Find the masks. Return the island to its glory.
And later we will learn that the island was really Artahka’s home. When the Great Spirit Robot died, it was moved, by Artahka using the Mask of Creation, onto Spherus Magna for a new home for the Matoran. (Cause’ the Matoran don’t like the Agori 
)
Then the spiders came, something happened and all that remained were the Defenders and the Skull Spiders…
suddenly… ‘I am a machine’ starts playing…
Bravo, good one.
I’d agree with most of what’s said, if not for Pohatu’s artwork.
He has planetoids of some kind in the left corner of the box. So what if the masters a shot onto a planet where they have to get the masks and fight the spiders, the planet being like the Throwbots in that it contains many elemental locations.
I actually never noticed the planets. I suppose they could be highly significant to the story or mean nothing at all.
Hopefully having the planets could still get us in the later years some '04 vibe going with more futuristic stuff too?