I'd argue that there are still a few retro reps that could be considered more likely than any others.
Takamaru has been considered many times throughout Smash's history, and it might finally be his time. He's finally had an international release of his game on the eShop, and he gained some exposure from his roles as an Assist Trophy in Smash and as a game in Nintendoland.
Excitebiker has been considered before, but was dismissed in favor of Ice Climbers. However, the franchise is well-known and recently received a course in Mario Kart, alongside names like Mario, Zelda, Animal Crossing, and F-Zero.
Balloon Fighter was considered in the past, though ultimately was discarded in favor of Ice Climbers. His game had a stage in 3DS, and the Villager was given an up-b that referenced him. While I don't expect him, Sakurai could always come back to the idea.
Sheriff was included as an assist trophy and may be considered for a playable role. After all, he is arguably Nintendo's first playable character, and is the first project worked on by the duo of Miyamoto and Takeda. Apparently,
Miyamoto has fond memories of the game, too. That said, the character may fall into the realm of a historical character ala Mr. Game and Watch, R.O.B., and (arguably) Duck Hunt, rather than the realm of retros like Ice Climbers, Pit, and (arguably) Little Mac.
Sukapon was considered as an item in Melee, but was scrapped due to "adult matters" that likely had to do with rights. He has been requested a fair amount, which might bring him onto Sakurai's radar, especially if rumors that the copyright holders went bankrupt have merit.