By world, I mean Termina. If the word "land" works better for you, then so be it. As far as I'm concerned, all we know about Termina is the region that was playable in MM.
I don't think they'd suddenly just make another timeline
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Ocarina of Time had a good reason to split the timeline in two, but remember, that rule was established after TP was released, after Aonuma had figured out its placement. I doubt they'll split it again soon, because frankly, there's no event as huge as the one from OoT that could warrant a split in time.
Well, could Navi potentially awaken as the Fairy Queen? I'm sure a Fairy Queen is not just born as a Queen.
But yeah, this "Sword girl" does represent the TP Master Sword very much so, but let's look at it from this angle. The Master Sword in OoT is the same as seen in both WW and TP, but both of those iterations look considerably different from one another, mostly due to the art styles of their respective games. I find it very intriguing that the art style of OoT/MM is like a perfect blend between that of WW and TP. Now, Zelda Wii is supposedly being crafted under the same/similar design as TP, but we know officially that it has nothing to do with TP, BUT, because the sword-girl's (let's call her Master Fairy) design resembles the TP Master Sword, it's obvious that there's a connection. Given that the artwork does not depict a more developed world, this is probably not based after TP in its timeline, so there's a great chance that it's based beforehand, and hence, further corroborates the theory that it is a sequel to Majora's Mask.