In games like Call of Cthulhu and Delta Green you play investigators learning of the hidden reality and losing sanity as you do so. Eventually characters go mad in a variety of ways.
But an idea that appeals to me, having read many crazy things on the Internet and seen people commenting that crazy under news stories, what if you don't just shriek in a corner or hear voices?
What if each "madness" you get is actually that you start believing a pants-on-head level of conspiracy? Once you begin accepting there is a dreaming ancient monster in a non-euclidean city on the bottom of the ocean or that invisible aliens steal brains, or Troy McClure is part of an ancient people with many undersea cities... you might start to lose your grounding and believe other ridiculous things. Things like...
01.) The government is planning to impose martial law in Texas with a military operation..
02.) A secret conspiracy is hiding evidence of a UFO crash in New Mexico..
03.) There was a second gunman..
04.) Jet fuel can't melt steel beams..
05.) The moon landing was a hoax..
07.) David Icke's reptoid theories..
08.) FEMA is turning abandoned walmarts into concentration camps in waiting
09) the Bilderberg group is trying to impose a new world order
10.) So is the UN!
11.) something something the Illuminati.
12.) The Masons are hiding secrets about the nature of the world
13.) So is the Vatican.
14.) And somehow the Elder Scrolls of Zion make sense to you now (worst game in the series)
15.) Vaccines are a plot to cause autism
16.) Psychology is a scam to hide the existence of thetans.
17.) The Phantom Time Hypothesis is correct
18.) HAARP is in full swing
19.) The simultaneous 4 day night cycle is correct, and being hidden by evil educators.
No, can't use that anymore, how about
Nope, ok, how about
20.) The Shaver mysteries are real
The idea being, that as you are forced to abandon so much of what you held to be true about reality, its very easy to pick up and run with other things you once just brushed aside as feverish dreams. The more of these false beliefs you pick up, the less effective you are as an investigator. You will keep trying new information back to more and more red herrings. People who come to talk to you will hear you rant about more and more obvious bullshit before you get into any real supernatural knowledge and they will probably have tuned out by then and labelled you a crank.
And that is how your investigator is retired, not hearing voices, but rather typing in all caps in the deep bowels of a brietbart comment thread with another conspiracy theorist. Occasionally an active investigator will come by to your one room apartment that smells of cat pee and looks like an episode of hoarders. There they will try to tease out what you actually know about Shoggoths from what is your conjecture about Chemtrails and Mason protective symbols. Of course, your character has spent years doing mental gymnastics so there is a strong risk not only that they won't get anything useful, but worse that you might convince them of a false link (represented by SAN damage as you put a thought in the back of their brain).