Here's a interesting idea for you!
I was thinking about the Mask of Time, and how it seems to stand out a little bit too much.
I mean, it's a all powerful mask, too powerful to have simply been made by the Matoran.
And then it hit me.
What if, the mask actually belonged an ancient, all powerful being, one who stood alongside the Three Brothers (We're still going for three of them, right?)
I present to you, the pitch of Vahi, The Timekeeper, The Fourth Sister.