Personally I think there are two moves that could be potential dealbreakers for Dixie as a Diddy clone - the rocketbarrel boost and peanut popgun, since both are supposed to be Diddy's signature moves in DK64. Donkey Kong Country 3 had rocketbarrels but they worked differently enough to necessitate a completely different animation, and Dixie doesn't really have her own counterpart to the popgun. I've joked about this before but I think Donkey Kong has a lot of stuff that would make more sense on Dixie - up-special and neutral air come to mind. That said, he's also probably got even more dealbreakers, since a lot of Donkey Kong's moves rely on leveraging his bulk and the length of his arms, and likely wouldn't make sense when scaled down for a character with radically different proportions.
Of course I also think the Wario example from Melee shows that Sakurai would rather cut a character entirely than implement them in a way that he feels doesn't do them justice, so it's entirely possible that if Dixie hadn't been planned since early on, she might not make it in at all.
Personally, if she is a late addition, I'd expect a similar situation to Wolf in Brawl. Rather than a direct clone of one character, I could see her kit being largely made up of re-used animations from multiple characters with maybe one or two original animations.
As of Tropical Freeze, Dixie Kong now has the Bubble Gum Ball Pop Gun, so now she can share that move with Diddy Kong. The main thing that makes Dixie Kong differ from Diddy Kong is her pretensile hair. I believe most of her trophies throughout the Smash Bros. series make mention of her using her hair to fly, pick up objects, and attack enemies.
Also, its interesting that you made the comment of Dixie Kong being an amalgamation of multiple characters. I thought years ago this might be a possibility for Dixie Kong too. Her recovery could work similarly to DK's, and she has a ground pound in Tropical Freeze like DK does (only she uses her hair). She has a pop gun like Diddy Kong does as well as a similar body type. Its certainly possible that Dixie Kong becomes the "Wolf" of Ultimate (Wolf was the very last character added to Brawl).
Its possible that Dixie Kong started development as an echo fighter, only for Sakurai to have second thoughts and develop her as a semi-clone. Lucas actually started out as a Melee style clone before being developed into a nearly unique fighter. Dixie Kong not being able to use her hair for a few attacks, would not be true to Dixie Kong.
My personal prediction is that if we do get K. Rool, there is a good chance Dixie Kong will be following (if not, then Funky Kong as a Donkey Kong echo fighter). Dixie Kong might either be an echo fighter (due to her similar body type as Diddy Kong) or a semi-clone to Diddy Kong. There is the possibility that she ends up a Lucas or Wolf style derivative fighter, or perhaps an entirely unique one, however, given that there is limited time to add additional newcomers, I lean towards echo or semi-clone (think modern Falco, Roy, Ganondorf). I feel optimistic on K. Rool, due to the characters likely strong ballot performance the world over, so there is a decent chance for Dixie Kong as well. Dixie Kong could just as a easily become Ultimate's Captain Ganondorf as she can become Ultimate's Wolf.
We have only two true newcomers known thus far, so it is actually quite difficult to ascertain what Sakurai's criteria this go around is going to be. Ridley and Inklings are both popularly requested characters, but one was extremely obvious that they were going to get in the next installment (Inklings) while the latter it appeared to be that they might never happen (Ridley). Ultimate releasing only four years after Smash 4, with the interim epoch being the twilight years of the Wii U and 3DS, makes for an interesting scenario in terms of what decisions Sakurai might go with for Ultimate.
Moreover, there is the matter that Sakurai has already warned us "not to expect too many new characters", which to me at the very least means less newcomers than Smash 4's base roster. Smash 4 had 15 newcomers, 13 if including all three Mii Fighters as one, and 17 if including the two echo fighter newcomers. I personally see 8 additional non-echo newcomers as the limit for what to expect for Ultimate's base roster. The lowest number I expect for non-echo newcomers is five. So 5-8 additional fighters. Just with this sort of range speculation can wildly differ as to who makes it and who does not.