Here is the Stadium, which could be used for the battle arena stuff
Of course the brown part (dirt) is not finished. It should be built into an arena (or possibly multiple different saves with different arenas)
But the stadium itself is done.
Also here's some concept I came up with that could work on a no-gravity track
The regular Speedkarts sort of "stick" to the ramps, so you'd be able to park your kart on a ramp and it wouldn't slide down (thus its drive-able)
And lastly, there's a short track I built,
Emil Circuit (though the scenery is not quite finished)
(go on, sue me!)