pieces should be able to be blown off
Hopefully interactive. Imagine the new lighting being used as headlights / signals. I don't think this would be too hard to do. Look what happens when you blow a wheel off of a car in game. It is solid as a rock until a wheel is somehow remounted. Once clicked, the wheels should mount and the car cannot be edited after this.
vehicles should be done like in a ripped off lego digital designer.
it can be a second "game" to play from the main menu. it should have like dozen different preset base-bricks with wheels already attached. and you simply build on it using the normal bl palate. seats are predetermined with a hitbox to preserve space for avatars.
once done, it should be auto saved as a static vehicle model, and be able to be used in game.
/my crazy idea
If you look at Lego Universe, they just open a window to design a vehicle. You design it, then its yours in game.
In Blockland, you should be able to use a hotkey to open a secondary window, and using default bricks, design a car. Once done, your car will spawn and you are set to go. Because they are default bricks, no one downloads anything.