I had some thoughts about the Call of Cthulhu Cthulhu Mythos skill while listening to some podcast or other - I can't remember which one now. Possibly RPPR. You can read a tome about Hastur and suddenly know about Ithaqqua. Or you can sometimes know about byakhees, and sometimes not. It's weird.
An idea came to me that you could introduce a fractal approach to Mythos skills (and indeed others, but let's stick with Mythos for now). This would be a tweak specifically for games where there's quite a lot of Mythos going on, and particularly suitable for long-term campaigns focused on a subset of the Mythos but including elements from other factions.
As it's Call of Cthulhu specific I've posted it on my Call of Cthulhu blog. But feel free to comment there or here as suits you.