Brand new is fine; I think mainly we need arenas of different sizes and difficulty (item/banana/nitro distribution), and I think experimenting with pushable objects could be fun (though don't have hundreds of those or performance will go down).
One idea for an arena I've had for quite a while is a track with a "Duplo"-like theme (don't use the brand though
). The general idea was to have two castles opposite of each other, a bridge going over from one to the other on the top, and a wall of movable large pieces under which block driving from the lower entrances to the other side. The wall could be broken by pushing one of the heavy boulders on the top of the castles down the ramp which lead to the lower entrance (both the sphere/boulder and the wall pieces could be quite heavy; ideally the wall isn't easy to break down without using one of the two boulders). And obviously some items etc would be needed, but relatively sparsely distributed.
Not sure how fun this would be in practice, but in my mind it seemed like it could be. Anyway, feel free to create whatever arena you want; as long as it's of the quality you've shown so far we'll certainly consider them (case-by-case obviously, when you have something ready).