"Minimal additions to Smash" eh? Let's assume the Switch version contains every stage from both the 3DS and Wii U versions and all the DLC. To bring in double- and triple-dippers, Nintendo surely knows they'll need to add some new characters. So, let's assume they add 6 characters. Why 6? Because it's only one less than the DLC character count. Let's logically figure out who they might be:
INKLINGS: Seems far too obvious to me that Nintendo would want to harness the popularity of Splatoon, especially in Japan. After all, Splatoon was one of only four first-party games shown in the Switch teaser, so Nintendo probably realizes how important the IP could be to them. Gameplay wise, they ooze the creativity that Mr. Sakurai loves so dearly; even a simplification of Splatoon's deep ink mechanics would still make for a truly unique fighter! The costumes would work like Villager, half boy, half girl in various colors. The Inklings would bring a Splatoon stage with them; perhaps Inkopolis?
ICE CLIMBERS: A highly requested veteran that missed out on the 3DS/U games due to technical limitations. Why not show off Switch's increased power by bringing back the duo?
NAMCO REP: Capcom has two reps, as does Sega. Why wouldn't Namco, who co-developed Smash 4 get an additional rep? Most likely, this would be a Tekken character, probably Heihachi. If I'm not mistaken, he was considered for DLC anyway. Regardless, this Namco rep would bring a stage along.
RHYTHM HEAVEN: A series icon for Rhythm Heaven was datamined shortly after the final DLC characters' release. Presumably, if the Gematsu leak was indeed legitimate but outdated, a character was in consideration at one point. If this were to happen, I would personally expect Karate Joe, as he is one of very few characters that have appeared in each of the four Rhythm Heaven games. This rep would bring Rhythm Heaven itself as a stage.
KING K ROOL: Given the massive rally for his appearance as DLC, and the subsequent, conspicuously timed Mii Costume, one can only assume that Nintendo is aware of his popularity. Provided that he did indeed catch Nintendo's eye, he would be a solid addition. Mr. Sakurai would surely see quite a lot of room for a creative move set in K. Rool's history. He would bring Gangplank Galleon as a stage.
RIDLEY: The showstopper. The big bombshell. The sheer shock factor of Mr. Sakurai changing his mind on Ridley would hype up many, many fans. Ridley would not require a stage, but the Pyrosphere boss would not spawn if Ridley is fighting.
What do you folks think? Shoot some suggestions! Try the 6 character format!