We've got one Twilight Princess stage, Bridge of Eldin, and while it was a fairly good choice, there were many better. But with two TLOZ characters in Brawl so far, there's room for at least one better Twilight Princess stage. Possible stages...
City in the Sky
Awesome. Not only would there be plenty of wind elements, there'd also be Argorok, the boss.
Goron Mines
Especially the main room, the Volcano Forge area. Brilliant.
North Faron Woods
I'm talking about the exterior of the Forest Temple here. Giant tree would be fairly cool, plus they could add in some of the hazards on the path to the Sacred Grove.
Palace of Twilight
That would own, considering all the terrible beasties flying around in the darkness, and the veils of darkness.
Sacred Grove
The Skull Kid, plus the whole area is fairly confusing, so that could be an element.
Snowpeak
A stage on a snowy mountain with blizzards? That would be awesome.
Snowpeak Ruins
A castle stage. I could've put Hyrule Castle, but that would've been boring. Here, there's slippery ice surfaces, Freezards and cannons.