The air city minigame with friends was the most memorable part. Similar to the mode in Smash 4 for the 3DS where you would pick powerups before the main battle against a friend began.
With an emulator it's actually possible to play the gamecube game online now. There is an active discord server where they occasionally host city trial tournaments.Though I haven't had much time to look into it, so I am curious to see what Nintendo will do with the new entry and if the city trial returns.