Impa would pretty much have to be based on Skyward Sword since that was the game she actually did the most in. Admittedly, that would not be the worst thing ever.
As far as one offs go, I'd say Midna actually. Yeah, Skull Kid has a following, but Midna should get props for being perhaps the only Zelda partner to actually be liked by majority of fans. That, and she should have a decently varied ability set if you can merge her own abilities with some form of wolf whether that be Link or they go a Hyrule Warriors route and have her conjure up her own. Add this to taking some liberties and making new moves, and you have yourself a unique and well liked newcomer.
As far as one offs go, I'd say Midna actually. Yeah, Skull Kid has a following, but Midna should get props for being perhaps the only Zelda partner to actually be liked by majority of fans. That, and she should have a decently varied ability set if you can merge her own abilities with some form of wolf whether that be Link or they go a Hyrule Warriors route and have her conjure up her own. Add this to taking some liberties and making new moves, and you have yourself a unique and well liked newcomer.