More tracks and more cars, certainly... but when it comes to game engine eliments being changed like weather, I could see it getting tricky and out of hand fast.
I suppose you could make any game modular enough to accept almost anything via patching, however I am pretty sure that would take a lot of extra work up front and eliminate potential performance related shortcuts and trick that rely on knowing what is going on elsewhere to avoid doing unecessary work.
For instance everything I have worked on has benefited from one module knowing how another module will work, knowing what it might expect and what it knows what kinds of arguments will never be passed.
If you have to make every module ready to accept potentially anything from another module that might change who knows how would have almost always had some drastic results.
