The way i see it is if Dixie is a tag team, she could be added as a similar model to Diddy. basically, she would have the same normal attacks, but her specials could involve hair tethering, more horizontal recovery, etc. Pretty much, just adding to what Diddy already has, not completely revamping. The idea of a tag mechanic wouldnt completely change how he plays, rather add to what he has. Similar to how it works in DKC, they both run, jump, swing, etc. but there are certain things that make them unique, such as Diddy's speed or Dixies ability to hair glide.
And have a little faith in Sakurai. I didn't like the idea of Rosalina or WFT, my heart sank when I initially saw them. After seeing some gameplay, i instantly turned my opinion around and thought "that is not what i expected at all." It could happen with a Diddy/Dixie tag team as well.
Not to mention, in context of changing characters, have you ever heard of Luigification? Falco got some, Luigi got some, Ganondorf got some. Granted it wasn't a huge change, but a change nonetheless