I've seen the same pattern in many different fields: even though
lots of people have worked hard in the field, only a small fraction
of the space of possibilities has been explored, because they've
all worked on similar things.
Even the smartest, most imaginative people are surprisingly
conservative when deciding what to work on. People who would never
dream of being fashionable in any other way get sucked into working
on fashionable problems.
If you want to try working on unfashionable problems, one of the
best places to look is in fields that people think have already been
fully explored: essays, Lisp, venture funding — you may notice a
pattern here. If you can find a new approach into a big but apparently
played out field, the value of whatever you discover will be
multiplied by its enormous surface area.
The best protection against getting drawn into working on the same
things as everyone else may be to genuinely
love what you're doing.
Then you'll continue to work on it even if you make the same mistake
as other people and think that it's too marginal to matter.