Open world Kart racer like Forza Horizon with Karts? LOL. Could be cool if it has a Splatoon like aesthetic.
Also have things from Sonic Racing Transformed like air/land/sea capabilities plus mods.
Shared world, too.
Given how great the Sonic Racing games turned out I'd trust Sumo with some design, but Playground has proven they have skill when it comes to designing for open world.