In the 2016 Wii U version of Mario & Sonic, Zavok is a Guest character, which means that not only is Zavok a boss, but he's also a playable character specifically for an event only (in his case, Boxing). On one hand, this is a step up from previous Mario & Sonic games where bosses were only meant to be battled against meaning that them being playable is a major step up, but on the other hand, fans of certain characters are bound to be disappointed that their favourites aren't majorly playable. Toad for example was only playable in 100x4 (baton pass relay race), and I am sure some would be disappointed if that repeated here too. I am pretty sure Rosalina fans would be disappointed if she were once again restricted to Gymnastics again.
Speaking of this, I am super happy that the human characters get to wear sports uniforms instead of being stuck with the original clothes, which applies to all seven human characters (6 from Mario + Eggman) and I think every Sonic animal character. For some reason Bowser, Bowser Jr. and Yoshi couldn't wear uniforms (not sure about DK). Some uniforms even have characters being barefooted, which is a nice touch since some events don't require shoes (such as swimming). Fencing is another example, as I caught a glimpse of Waluigi in a nice white outfit.
Thank you for reading.