Well, for Impa, I imagine her being a cross between Sheik and Zelda. But honestly I do agree, I'd have rather had Sheik replaced by Impa outright.
The character I'd really like to see from Zelda has no chance of ever making it so I never really brought it up much, that's why in general I kept suggesting Impa more than anyone. I'm just happy that my favorite Zelda character was the only OOT 3D trophy added to Smash 3DS that wasn't a recurring character. I always thought she was one of the more important and memorable characters in OOT and that kinda helped cement it even more for me.
It'd have been somewhat more realistic if she made it into Hyrule Warriors... which honestly I thought that was a sure thing, especially when we got Agitha of all characters.

Well, there's still DLC, but until an OOT DLC pack is announced I'll still be waiting anxiously, hoping ever so slightly in the unlikely scenario than a non-TP/MM character is added in those packs...
Lana is actually semi-popular, a lot of people do like her. But she's got a huge mixed reaction in general. Although its mostly for stupid reasons, namely her feeling like she's too "anime." Honestly, I'm kinda disappointed they went with this storyline and didn't include one of the biggest characters who would fit into such a storyline... I've heard people comment on how said character maybe just "didn't fit" into the story they were going with, and I fell for it at first. Then I took a step back and realized "WTF am I thinking? She'd fit into this storyline PERFECTLY."
Either way, doesn't matter much. Hyrule Warriors isn't likely to get represented in Smash Bros. at all, let alone a character exclusive to HW. I mean, yeah, our expectations aren't always accurate, but I'd say in general most people would agree with the notion that Lana is extremely unlikely.