Did they really just take the same old Twi vector from three+ years ago and shop some wings onto it?

if you notice on the tree in the background you can see celestia's and Luna's respective cutie marks.
what if they are spirits of harmony, like how discord is the spirit of disharmony, that once dwelled in the tree. But when the ponies needed help the most, they came and took care of everything to make sure harmony was sustained. This would also explain why they never aged. They'll disappear thinking that their duties to equestria are complete and are giving the reigns to Twilight and Cadance.

also note it seems to be in a cave… what if that was the cave during the Hearth Warming Eve story, the site where the fires of friendship fended off the Windigos. they sewed the seed that became the tree.

well there's my wobbly headcanon, doomed to be shot down during the premiere