I've given my opinion on alternate skins before but I suppose I will restate it.
There are some characters who are very similar and would be a waste of resources to include. I see this with Diddy/Dixie as well as Fox/Falco/Wolf etc. In order to please as many people as possible one simple solution would be to include these characters as alternative skins.
For example, have Diddy as the main character but be able to switch to Dixie which has a different aesthetic but plays the same. We saw this with the Male/Female Trainer characters. Likely they will just be alternate skins of each other and to reduce clones but keep everyone happy, it should be done with other characters such as Diddy/Dixie and Fox/Wolf/Falco and Multiple gens of pokemon starters for Pokemon trainer and Dr Mario and possibly some FE characters...