If that ever happened during development, she would have been upgraded since Lucina was.
I'm saying that pretty confidently. They're both popular, but Dixie's more popular, and she's recurring in her series, and the major reason- DK representation is low and Fire Emblem representation is high.
I'm not saying that even using the series' merits.
Fire Emblem literally has more representation than DK, even if Dixie was to also get upgraded.
It got a unique newcomer and a new 3ds stage... and a new Wii U stage, but DK will get that much eventually.
Anyways, I don't think she was planned as an alt, but I'm not considering it impossible.
She'll probably either be a Wolf/Luigi-esque version of Diddy (as in, barely a clone, even though they use many of the same moves, it requires some different techniques, and some moves are completely different)
Or a unique character based off mainly her hair. I'd like to see some items and other references, but I'm not counting on it.
I do think she's still likely, maybe more just because she's always been a major contender and not because of leaks.
But there is one thing I think that I haven't seen brought up, even if Dixie is unique and has her own specials and hair attacks and all that, she should still take Diddy's Banana! Instant top-tier!