Listening to the Innsmouth House Players roll up characters for Shadows by Gaslight (a YSDC patron only recording, hint! hint!), I spotted another candidate for CoC house rules.
Even though I have been looking through the editions I own, I haven't found any trace of the rule they used, that you can't start with a skill higher than 80% or two at 75%. I am grabbing those numbers from memory, so they might be slightly off. I like that idea, since it leaves some room for growth even for the "experts".
Related to that, I am personally not that fond of the idea of gaining 1d10 percentiles when increasing a skill. Way back it used to be 1d6 and I think I prefer that. Like in Traveller, "real world" characters should start competent and then grow slowly. It just makes more sense.
Finally, in Unknown Armies all characters have one thing called the "trigger event", which is what made that character aware of the Unnatural. Tied into the Drive, I think it would make excellent sense for CoC PCs to define such a thing. I noted that the Keeper of Arcane Lore for Shadows by Gaslight, Helen Maclean, used such an idea to provide some initial Cthulhu Mythos skill to the Investigators. Me like.
Now I not only have to collect these and other ideas in a document, I really should get a group together to play CoC!