They could, but they've also allowed, for the first time, Mii Costumes of Pokemon Characters to be sold as DLC(The Team Rocket Grunts).
As it stands right now, at least in regards to Mii Costumes, they seem pretty okay with DLC.
Yeah, which is why I think that TPC is just unpredictable.
Also, if I recall correctly, Nintendo gave Sakurai a list of characters to choose from, I could see him opting for someone he found more interesting over a new Pokémon. Considering that every newcomer thus far, except for Byleth, is from a new IP being added to the game, I could also see him deciding to go for more new IPs than one already well represented..though Byleth still got in so clearly it won't be the case every single time, its just a 5/6 case atm..2/7 if you count Piranha Plant, who almost seems like a case of just not being finished in time for Base or being held back due to not seeming 'big' enough for base and thus was given away for free..considering he has something none of the DLC got, Palutena's Guidance.
He also seems like he wants each DLC character to have some unique gimmick..
Joker - Persona mechanics
Hero - Random Crits, Swapping Spells
Banjo-Kazooie - Rival Console Dev character, Wonder Wing
Terry - 4 Specials, Secret Specials
Byleth - Many different weapons
Min Min - Directional ARMS, Tap vs Hold, ARMS Change, Aerial Smashes