Has the whole Lucina thing not changed anyone's opinions on Impa..? In spite of loving the character I didn't want Impa to begin with really, I always liked her but preferred Tetra. But now...
1. It ties in with the Trophy theory, whatever you reckon that counts for.
2. Easily bump-uppable from costume to fighter, like Lucina - yet more visual distinction from Sheik than between Lucina and Marth
3. Another badass lady fighter on the roster.
4. Has appeared in the most amount of Zelda games after Link, Zelda and Ganondorf.
5. Was the /first/ character announced for Hyrule Warriors, and heavily promoted.
6. As of point 5, she has now had a major role in FOUR of the pat five Zelda game: Skyward Sword, Ocarina 3DS, Link Between Worlds, and Hyrule Warrios - all bar Wind Waker. Huge presence in the current handheld and previous home console generations - the ones that typically influence Smash most heavily.
7. Now that Nintendo are paying more significant attention to competitive scene, they'll be aware of Sheik's popularity and perhaps consider that will warrant a clone - much the same as Lucina with Marth.
8. Representing Skyward Sword without having to use a probable one-shot character. Also, including someone from SS would leave all the major recent Zelda releases represented.
9. Finally, like TL with Link and Lucy with Marth, their similarity in playstyle would be justified. Impa is the one in Ocarina of Time who trains Sheik - Sheik literally learned everything she knows from her.
If anything, she's become one of my most expected.
(I posted this in one of the Sheik threads, thought I should post it here too)