Here are two ideas for gamemodes, aside of race-FTL. These are only suggestions, so I'm not going to get offended if they are implemented or not.
First is the Dynamite mode. Every car starts with Dynamite on their back, and must reach checkpoints and/or complete a set of laps before the dynamite goes BOOM! They must not slow down, or otherwise, the bomb explodes. After every lap/checkpoint is reached, this low speed limit is increased. If the dynamite explodes, they're eliminated. Here's an example of the mode:
http://www.youtube.com/watch?v=dypCJDAKgxk
Tracks where this mode can be implemented: Green Valley, Shiny Suburbs, the newer Minigolf track.
Second is the Stunt mode. In Stunt mode, the karts should do all kinds of tricks (nitro jumps, mid-air spins and rolls, etc) to gain points. The one with the highest amount of points (or the one who reaches the point limit) wins.
http://www.youtube.com/watch?v=ZlBxonr5CQE
Sadly, there should be maps made specifically for this mode.