Stack Exchange sites feed off expertise. It's not practical to build a quality Q&A site that vaguely covers an infinite variety of disciplines. At the very least, you'd have to partition it out into sections for each discipline. You know, like sites.
There are a lot of questions out there that will never be a good match for Stack Exchange's model. Questions that just don't make sense or are too broad, for example.
For the questions that would be a good fit, but don't belong in any existing sites, I'd argue that there is a somewhat finite set of site-level categories you could vote for on Area 51, that would give you 90% coverage.
Just take your questions up a few levels. Instead of wanting for a site about handing nulls in C#, stretch out the scope into a programming site. Instead of one about late-80s BMW engines, try for automotive maintenance and repair. If you take out the scope a bit, there will typically be a point where you encounter a viable site-level category that contains a number of questions, but is still small enough to build a strong community. That's when you should post something on Area 51.