UPDATE 15/6/21, 11PM: In much the same way that Sonic made a surprise roly-poly appearance in Super Monkey Ball: Banana Blitz HD, it looks like the blue hedgehog will be making a playable return in Super Monkey Ball: Banana Mania - and this time with Tails in tow.
One enterprising internet sleuth (as spotted by Sonic fan site Sonic Stadium) has gone a-poking around Sega's newly launched Super Monkey Ball: Banana Mania website and - presumably through a combination of curiosity, cunning, and guesswork - uncovered currently hidden art for a potentially playable Sonic and Tails.
You can see for yourself! Scroll down to the Characters section and you'll find six boxes, each showcasing one of Monkey Ball's familiar playable roster. Conspicuously though, the site layout leaves enough room for two more characters - and manually altering the image paths so portrait and in-ball art ends in sonic.png or tails.png reveals hidden Sonic and Tails versions of both: