https://imgur.com/TMZj3oZ
(not my picture)
Not to get your hopes up.
But there might be a chance for them to be playable in the future.
Seeing how we also got references to Nina, Brio, Rilla Roo, (King Chicken) before they were actually playable.
Referenced characters in the tracks that are yet to be playable:
The Twins, Madame Amberly & Evil Crash (Nina's Nightmare)
Rusty Walrus (Megamix Mania)
Your vote: Option 4: They won't be added to the game.
Redditpoll.com