Personally, I don't see what's the big deal with replacing Sheik with Impa. I mean if she plays like Sheik but only looks and sounds like Impa I don't see where there is a problem. Since there is a large disliking about it I believe she has a lot to offer as a separate character.
Impa has been featured in three main forms throughout the Zelda universe.
Young Impa from
OOT and
Skyward Sword
Middle Age Impa from the
Oracle Games
Old Impa from
Zelda 1,
Adventures of Link, and even the
CD-I (There are more but they would be consider spoilers)
If you really wanted too you could make a different character for each of these models.
Personally I like the young Impa the best and even though she's very similar to Sheik I believe there are plenty of ninja moves/magic to go around. Since Sheik never used the chain whip or the spike in game I think Impa can have some added moves too. I'm thinking she could a mixture of hand-hand and weapon style fighting (similar to Snake). She could pull out her Kodachi (the weapon on the back of OOT Impa) for smash attacks. I would also love to see her use a
Kusarigami in some attacks or specials.
Middle Age Impa would be a bit comical as a big power house woman. She would be strong and heavy similar to DK. She could also have magic to assist her like modified moves from Skyward Sword.
Old Impa which is the most featured would be a mana-cannon character. She would use magic to move around and perform many of her sky attacks. Her frailty is offset by her magic prowess. Now you may say she's too old to fight but there are plenty of old aged characters in fighting games. One example is the fellow ninja Splinter from TMNT Smash Up. Plus, you put your opponent in a guilt trip of beating up an old woman.