I wouldn't mind having Yoshi in this game. Why not?
I don't get why people hate certain Mario characters. I mean how? What did they do to you?
The only characters I hate are the ones that are the same character coexisting at the same time. Like Dry Bowser and Bowser being on the same race course... WHAT? See, I don't hate Dry Bowser. I only hate it when they consider him his own character. He's not his own character! He's Bowser!
Anyway having more playable characters never hurts. I think Nintendo is starting to accept that by adding Peach instead of Yellow Toad again.
My list of characters would be this:
MARIO
Pros: Balanced Stats
Cons: No Special Abilities
LUIGI
Pros: High Jumper
Cons: Poor Traction
TOAD
Pros: Fast Runner
Cons: Low Jumper
PEACH
Pros: Can Float in the Air
Cons: Slow Runner
WARIO
Pros: Can Ground Pound Shockwave by Himself
Cons: Slow Runner and Low Jumper
DONKEY KONG
Pros: Can Climb Up Walls Without Catsuit (but not as far)
Cons: Power-Ups are Weaker (Fireballs move slower, Boomerangs don't go that far, etc.)
Okay all I added was Wario and DK but I can't really think what other character to add that would have some unique advantage or disadvantage.