We still don't know what he's thinking personally, if he sees fan demand for certain characters and decides to promote Waluigi or Isaac for example, he could still do it if Nintendo agrees and can still represent more games (even with Waluigi, some older Mario sports games and DDR aren't in Smash at all, and already-represented worlds don't even actually have the world or most music). What you're saying could also go for other forms of representation, including trophies in Smash 4. I don't see why he can't reconsider at least one or two characters between DLC seasons when he still has most spots left for 100% new worlds and has already reconsidered characters between games, including between Smash 4 and Ultimate with Ridley and whoever (Ultimate's project plan being finished not long after Smash 4 DLC ended). And they still chose Banjo and Kazooie, who got in thanks to Minecraft bridging the gap, Rare starting the talks, and Nintendo and Sakurai telling them yes.
Also, I really don't see why he would reject a character because "no, I don't like them or want them playable." Besides, based on what he said about Byleth, he would've preferred a different character.