I doubt Zazz would be a boss, because:
-Bosses were considered spoilers in previous games. They weren't shown off pre-release, certainly not in the first trailer. Especially true if this game has a story mode, like the handheld versions of Vancouver Winter Olympics and London Olympics.
-Table Tennis doesn't seem like it would be fitting for him as a sport. He's more of a speedy, crazy character. If he was a boss, he'd more likely be put into something speed or power based (he's pretty strong, as he can throw that moon thing around in Sonic Lost World). The fact he's playing table tennis makes me think he's just a regular character.
As far as the game itself, I'll consider it. Rosalina being added would help, but I REALLY hope they bring back the Adventure Tours mode from the Vancouver Olympics on DS, maybe make it on the Wii U version too. It was the best thing the series has brought. It was like a little sports RPG and the characters actually had fun interactions and quests. That game remains the most characterised Daisy has ever been, and it's a SEGA game.
EDIT: As an aside, I love the artwork for this game:
Mario and Sonic are
SWEATING.