I've shared this opinion before, but I definitely think Inkling and King K. Rool were saved for Ultimate, with the Mii Costumes acting as consolation prizes for the time being. You could say the same thing about Isabelle and Chrom in retrospect, but at the time people didn't really bat an eye at those two getting Mii Costumes.
I have a feeling that, if Smash 4's DLC continued past the Ballot and Bayonetta, third-party characters would've still been the focus. On the first-party side, Wolf is probably the last veteran they would've brought back, and I could see one 2016 promotional pick, probably one of these options:
- With Rhythm Heaven Megamix having a really late localisation (perfect for promotional picks, if Corrin is anything to go by) they could've returned to the Chorus Kid concept.
- Paper Mario is an established series, and with the character appearing in two games that year, it might've been the best time for him.
- BOXBOY had a second installment that year, with the third coming in 2017. If Sakurai wanted to get a little crazy, Qbby was there for him.
- Kirby Planet Robobot released that year, and, well...