If the speculation about the "sword girl" is anything, it may involve the end of the Master Sword, or the breaking and repair of the Master Sword. When Ganondorf was killed in WW, the Master Sword was lost, as if it was no longer needed. When Ganondorf was killed in TP, it was put back on it's pedestal, so it must have at least one more story left in it.

The girl on the poster, whether it is the MS personified or not, will probably be Link's partner in the game. I suspect they will be building a larger and more advanced Hyrule than that seen in TP. They will probably save the kingdom from a terrible villain, maybe even a resurrected Ganondorf (I'm sick of that guy). I am fully prepared for the likelihood that Link will traverse a few dungeons, the main plot will flip, and Link will then traverse a few more dungeons.