If we were to have some sort of new Yoshi character, I'd probably give that role to Kamek (Provided that in future Yoshi games Kamek actually does something more meaningful than buff an enemy into a boss fight), but to me, I'd say Kamek could easily be fine as an Assist Trophy.
As for what third-party we might see next, I'd say that we might see a Western third-party newcomer alongside a Japanese one in the next Smash Bros. If you ask me, I'd probably go with Rayman or Doomguy. Rayman probably has a fair bit going for him, since Ubisoft seem to be on good terms with Nintendo - even going out of the way for making Rayman his own trophy in Smash Wii U. Doomguy could get in as part of Bethesda's current support for the Switch with Skyrim, DOOM and Wolfenstein II on the platform, so I can imagine him getting in eventually, but probably toned down enough for an E10+/T Rated game.