I mean, Metroid already has 2 newcomers so I feel like the series really doesn't need any more for this game. As for Yoshi, I feel like that series is far less separated from Mario compared to Warioware or Donkey Kong in that Yoshi regularly appears in main series Mario games and Baby Mario regularly has big roles in Yoshi games, so Kamek or the like would just feel like another Mario character, which we again don't need after Piranha Plant.
I think 1st party franchises that actually could use more reps would be DK, Kirby, Zelda, Xenoblade, or unrepped ones like Rhythm Heaven, whereas
Mario, Pokemon, Fire Emblem, and Metroid have enough as is.