Nice and original ideas

About the drunk driving, this would go in the same kind of post process system that could be used for SSAO

Some classics:
- auto-guided missiles
- missile towards the first kart
Actually, it could be interesting to have an auto-guided missile for which we could choose the player we want to hit, with some kind of quick and simple to use menu.
I think this could be really interesting when having multiple players hehe

A small detail I mentionned on IRC was that there should be different images for the plunger, according to the character we are driving.
Maybe characters could also feature special items: for example, it would be cool if Beastie's car could throw flames while Tux's kart could have "ice rays" on the side similar to what was in Rollcage (good old PlayStation 1...).
Here is the idea:

Those are just examples, and the problem with this approach is that is would make it "unfair" for the optional add-ons, which wouldn't have those...but it would give the characters much more personality I think

I really like the idea of hiker to be able to control another player's kart very briefly.
Also, when I played Crash Team Racing (a LOT), the karts could jump a little, and one of the uses of it was to get rid of the bomb attached to the kart. Currently in STK, when such a bomb is attached and we are far from other players (may it be because we are the first one or the last one), we just can't do anything about it and have to wait for our explosion, which is quite frustrating I think. We could hit the jump key a lot, and after sufficient trials, get rid of the bomb.
In CTR, bonus items had 2 "levels": when you got 10 apples (actually Wumpas

), the bonuses you had were more powerful (better accelerator, etc). I'm not sure if this really is worth it however.
+1 for the invincibility.
As hiker mentionned also, what makes the turtles in Mario Kart more fun than the bombs in STK is that they bounce on the walls, but that's more of a level design issue.