A different question.
You are in the mood for a documentary. On average, you reach for:

a) Something to do with space - astronomy or cosmology.
b) Something to do with world politics - current or recent.
c) Something to do with ancient history or the Bible.
d) Something to do with dinosaurs or other ancient life.
e) Something to do with chemistry or particle physics.
f) Something to do with a famous artist or artistic movements.
g) Something to do with alien visitations or psychic phenomena.
h) Something to do with geology - volcanoes, types of rock, etc.
i) Something to do with WAR - more likely the modern variety.
j) Something to do with the marvels of human engineering.
k) Something to do with modding cars and motorcycles.
l) Something to do with food - eating it or preparing it.
m) Something to do with travel - practical or dream vacations.
n) Something to do with psychology or psychiatry.
o) Something to do with other aspects of human health.
p) The good old fashioned African Savannah type (animals).
q) Something sports-related, a game, athlete profile, etc.
r) A biography, either a docudrama or the voice-over type.
s) Something to do with music, a band, a composer, whatever.
t) Something to do with ecology and climate change.
u) Something completely outlandish and like Crumb or Pi.
v) A making-of, behind-the-scenes dealie for movie.
w) Something about spirituality, religion, or philosophy.
x) Something to do with forensics - medical, aviation, etc.
y) Something to do with extreme weather/natural disasters.
z) Documentaries? Zzzzzzz.

You can just name one, or a handful, or put a few in order, or put the whole damn list in order. Or insert your own categories if I forgot obvious ones. It's your ball to run with. I'll add my answers when a few others have already done so.