I'm just gonna put this out here, and I am almost certainly going to get pretty flamed for it. I don't think Impa and to a lesser extent Dixie are as likely as people say.
Impa has a few issues that I feel hold her back from being that likely
- Repeatedly changing designs, the ones people are asking for were SS (As a replacement for Sheik because Skyward Sword Zelda would be taking the slot, then as a clone, then as a semi-clone) or Hyrule Warriors (Still technically a non-canon source for Zelda material). Unlike the differences in, for example, the Link or Zelda designs, this is not just a matter of art style adjustments but they have different body structuring and scaling. Between those two it isn't as notable as Link to Toon Link, but I feel like it's important to note.
- Her importance to Zelda as a series. Impa has always been a side character, in some cases only existent in the instruction manual, to help progress the story in one way or another, as Zelda's overseer. Her jump to Hyrule Warriors is about as grandiose as the jump for characters like Midna or Ghirahim who became assist trophies despite them having bigger roles in their games... at least directly.
- Relation to the other Zelda representatives. When we look at who has been chosen or at least planned as a Zelda character in Smash, we have 3 Links (Normal, Toon, Young). 4 Zeldas (Normal Zelda/Sheik, Toon Zelda/Sheik), and a Ganon(dorf). These characters have one big thing in common: The Tri-Force, the relic that the series revolves around. The fact that we have not had a non-triforce Zelda character yet at least sets a standard that I don't see Impa being large enough to break. Arguably, Ganon the Wizpig is more likely because then the Smash roster would have 2 Links, 2 Zeldas, and 2 Ganons.
- Where I've seen her popularity swell from. Legitimately, I've seen Impa's support become more pronounced at a few stages of the speculation for this game (Not saying it wasn't present otherwise, this is just when I noticed it being more open). First was the hype for her being shown off for Hyrule Warriors originally, followed by shared Ghirahim fans because of the Fi trophy in the April direct clearly having to mean a character (or people convincing themselves later on that this would prove Ridley via King Ki Hunter) and basically got a chunk of the remaining supporters from the Dazzling Sword when he was confirmed as an assist trophy. The last peaks for her supporters being more populous and vocal were the few on-cartridge, link-game, or DLC rumors that come every once in a while.
With Dixie, it is just the fact that the only thing that she has had to differentiate from Diddy are slower speed for more jump height, the helicoptering hair, holding objects above her instead of ahead, and appearing in less games as a less pronounced character. Her entire plan in Brawl was to be a tag-team which didn't work out.
I still believe Dixie is fairly likely for DLC, but I don't see the expectation for Impa. The desire, yes. But the expectation is lost to me. I just needed to vent this out a bit.