This is what I hear all the time. People are cool with Impa, but rather have a very unlikely character before her.
I get the appeal behind characters as Skull Kid, Ghirahim, Midna, Vaati, Masked Link, Zant or Wolf Link. I really do. All of them are cool characters, but thing is they are hugely irrelevant to the series overall. Also, many of them missed their chances majorly.
Skull Kid and Masked Link missed their big chance in Melee (Masked Link could work as the transformation character instead of Sheik).
Midna, Vaati, Zant and Wolf Link missed their big chance in Brawl.
Ghirahim will likely follow their example.
It's honestly only Impa who's important in more than 1 game out of all characters that have a fighting chance. That or Toon Zelda, however uninspired that would be. If anything, I'd rather have a solo Tetra to accompany Toon Link. Also, the potential of there being FOUR playable versions of Zelda is a bit... much. Zelda, Sheik, Toon Zelda, Tetra. Rather crowded. Rather have it be, Zelda, Tetra and Impa as all standalones.