Impa would be fine, but I doubt BotW or any games involving that version of Hyrule will be her ticket to Smash.
I know a lot of people argue that it wouldn't make sense to choose one Champion and leave out the others, but I think it would make even less sense to pass them up and add Impa instead. Both of the main games will likely use old Impa, meaning that the spin-off will be young Impa's only appearance. Then you have to understand that everything seems to point to Link, Zelda, and the Champions being the focus of that game's story. The absolute best-case scenario is Impa being the most important character after the latter six — a supporting character who helps you fight, but isn't nearly as crucial to the plot as the others are.
From a promotional standpoint, I don't think Impa is more likely than the Champions. The fact that they've featured the Champions on the boxart, made sure to give them lots of screentime in the initial trailer, and are reprinting their amiibo to launch alongside AoC speaks volumes to me.
For a while I thought the most likely newcomers for Zelda had shaped up to be Impa and Skull Kid. And from an "in general" standpoint I do technically still stand by that.
But timing is important, and given both this and Breath of the Wild 2, I'm starting to think one of the Champions may very well be one of the characters they chose for fighter pass 2 before the Byleth presentation aired.
Obviously only one of them though. If I had to guess, I'd predict that Mipha is the most likely, Daruk and Revali are somewhere in the middle (I guess I would say Revali is more likely if I had to pick between the two.), and while it pains me to say it since she's my favorite Champion, I think Urbosa is the least likely.
I will say, Urbosa has the advantage of being a more traditional fighter, and I feel like she'd be the easiest to work with.
Of course, you could also argue the opposite, which is why I'd probably rank the Champion's in the same order as you.