Disagree highly with your view on Impa. She has the potential to have a whole lot of different sorts of movesets even, unlike most other characters. She could carry a giant blade or naginata as in Hyrule Warriors, or fight as a sort hybrid of Zelda and Sheik as Skyward Sword Impa. She can cast a giant barrier to that is able to withhold attacks or reflect projectiles as shown in a cutscene in Skyward Sword. And as a time traveller, she could probably realistically use one of Skyward Sword's more interessting game mechanics ; the Timeshift Orbs. Deku Nuts are also a great possibility, seeing as Sheik doesn't use them, and they where removed as an item this time around.
So, big giant sword (actually, in Hyrule Warriors she even uses the Biggoron Sword) with water powered magic which she swings with great speed and agility, a naginata with fire powers and spear-like magical projectiles, time slowing attacks with the Timeshift Orbs, barrier to block attacks, Deku Nuts, magical ball attack, teleporting via Time Portals (making this up, but would work realistically- even if it's only her stage entrance), material arts based on kung fu / eastern fighting styles (as made up by her stance doing the barrier).
Videos of stuff:
*
Doesn't feature Impa, but the Timeshift Orb talked about earlier. SS Impa would likely get magical abilities that resemble this.
So there's basically a whole lot what she can do.
Just like Isaac, who's also a great choice, that I greatly agree with.