Mario Sports Mix details/screenshots, Slime confirmed
Wario Factory: Conveyor belt in a secret factory, items come down from the belt, compatible with all sports
Junior Street: Rich “bonus court,” more points when shooting from atop the bonus panels, can get 10 times the points in some cases, compatible with volleyball/basketball/hockey
Koopa Castle: Dangerous court, bubbles fly up from below the court, compatible with all sports
Waluigi Pinball: Pinball court operated by Waluigi Mecha, iron ball rolls around, compatible with volleyball/dodgeball/hockey
Ghost Ship: Sea-based court on a ship deck, Dry Bones comes out when boxes are broken, compatible with all sports
Table Western: Turntable court on a rail line in a wastelands area, trolleys toss bananas and coins, compatible with all sports
Starship: Sky-based court on an air ship deck, pick up crystals to possibly receive a bonus up, compatible with all sports
Nintendo has also confirmed on the Japanese Mario Sports Mix website that Slime will be playable. The character has a “tricky” play style and can summon other Slimes to build a tower for extra height as its Special Shot.
Thanks to Thomas N for the tip!
