the playgrounds needs a giant tunnel that spans around the track, filled with boostpads (?) powerups and obstacles to take the wall/celing driving to the max, think of a drivable CERN
this tunnel could have different sections in size, radius and wall angels to see how good transfers work at what speed.
i think most important as landvardaar already said is that the playground features different surfaces to test around with the games physics, several ovals , each made of different surfaces with small hills and bumps to see how the cars handel, how powerups , tunnels/walls made of differen surfaces handle and so on.
also jumps in differen heights/angels so maybe we can fight the biggest problem that RC had, random collsion when landing.
being able to give yourself any powerup, anytime in unlimited quantity
one specific thing i'd like to see would be huuuge drop.
like a single platform somewhere up in the sky that you can teleport to and just drop your car (and hopefully crash into something that blows up)
also physics toys like cones and rubble to play around :>