Some food for thought regarding all this "Dixie can totally be an Echo", or the new one "Dixie can be an Echo now because her Up-B can be changed". These ideas are thrown around simply because she CAN be an Echo, but not because she SHOULD be.
Just like
Parallel_Falchion
said, and I agree 100%: it is more than likely that none of the currently shown Echoes would've made it into the game as unique fighters. Yes, they are popular, and yes, they COULD be unique, but we must not forget that this game concentrates several important factors regarding a character, which include history with Nintendo, relevancy, etc. While these Echos have had roles in previous games, I honestly don't think ANY of them (bar perhaps Chrom in Sakurai's eyes) have contributed enough to their respective series to be considered for the game in a scenario where Echoes did not exist.
Think about it, Sakurai said Lucina was lucky she got in, and we can safely assume Dark Pit was added late to increase the roster number. Daisy, while an important character and highly popular, has never made a big jump like other Mario characters and be playable or have a major role in a bigger game other than spin-offs. Do we seriously think Richter would've made it as a unique character? Not with Simon Belmont's importance. Similarly, Dark Samus is not important enough to the Metroid series and is thwarted by Ridely as a main boss. Chrom got simply upgraded because Echoes are a thing.
The point I'm making is that all Echoes so far got in because they CAN BE echoes AND they are popular with fans, but certainly NOT because they are important and relevant enough to the Nintendo universe.
I simply cannot see Sakurai giving her the full Echo treatment, or have only her Up-B changed. Dixie can be an Echo and is popular enough, yes. HOWEVER, her contributions to Nintendo are of a different caliber, being the face of her own game and the co-star of perhaps the best game in the series, having a long history with Nintendo and being highly relevant due to the success of Tropical Freeze. She also did quite well in the Ballot I believe. She was also considered to be in Brawl, and I HIGHLY doubt she was thought of as a clone of Diddy there, as they were supposed to be a tag team. All other Echoes currently do not have these credentials. Dixie, in the worst case scenario, will be a Lucas/Wolf style of clone.
So please, think a bit before proposing the idea of Dixie being an Echo. Time constraints and roster size are a factor, yes, but I do not think they were a factor in deciding who would be an Echo back then in 2015: characters were put on the table and considered on their own merits. If Sakurai thought Wario was important enough to not be made into a clone back in Melee, I am sure he considered Dixie with the same eyes when deciding this game's roster, independently of who else got in.
TL;DR: Dixie is in a different category than all other characters who have got in as Echoes, who have only got in because they are popular with the fans but certainly not important or relevant enough in their own series to have made it as unique. There are some exceptions in the roster, and cases can be made for certain Echoes, but this is the main point I want to get through.