Philip Jose Farmer wasn't the first person to have the idea of bringing various characters from different stories. Such ideas date back to the stories of Jason & the Argonauts and King Arthur and the Round Table Knights. The juvenile dime novels would mix literary and historical figures with their fictional heroes. Indeed one that starred a Captain Nemo very different from the one from 20,000 Leagues Under the Sea and The Mysterious Island would be one of the things that Farmer would endeavor to reconcile by postulating the existence of no less than three men calling themselves Captain Nemo. One example of early cross pollination of characters in various media, in a 1940's tale of the Whizzer, the hero must bring the turn of the century safecracker and gentleman thief Raffles out of retirement.
Still, Farmer approached the idea with an almost near fanaticism of inclusion of every fictional story and character existing under one sky, with Doc Savage and Tarzan at the core. This passion is clearly echoed in the works All-Star Squadron and Young All Stars by Roy Thomas who brought in references to Fritz Lang's Metropolis, Edgar Allen Poe's The Narrative of Arthur Gordon Pym, and Philip Wylie's Gladiator. And, there's Alan Moore's League of Extraordinary Gentlemen. A beauty of Farmer's work is not so much that he created a story as much as a timeline and geneology and pseudo-biographies of Tarzan and Doc Savage, treating it with the eye of a historian and archaeologist. And people, continued with it after him, bringing in newer works and period pieces as well as science fiction television shows, characters from books in other languages, etc. While not all of the timelines can possibly be true, we have conflicting accounts of Holmes crossing swords with Dracula or meeting up with the Phantom of the Opera, but at the core is Farmer's timeline and geneologies. The timeline that I am presenting here is but one of the many Wold Newton based timelines out there, bringing in elements of various ones across the web, editing out some of the superfluous and much later accounts (such as the works in Marvel comics). There are a few other changes as well as I disagree or have my own theories that differ from the other historians (such as Holmes surely is not responsible for all the adventures attributed to him out there, a few must have been handled by other detectives that for some reason or another get attributed to the Great Detective).
Farmer's other influence on comics is less sure, it's my own conjecture. Farmer is widely attributed by his critics as bringing sexuality to science fiction. Not pornography per se (though there are sections of Greatheart Silver, that wouldn't get printed as a letter in Penthouse forum), but he brought an earthiness to the sterile genre, recognizing the baser instincts and drives that motivate mankind. "Behind every great man is a woman" is a saying that works on several levels. The need to impress and provide for, to preen and strut before those we are attracted to probably accounts for 90% of a man's endeavors. The first suspect in a murder is a spouse. Deeds great and small, good and evil, can mostly be traced back to the simple truth of that there is someone we want to have sex with.
Even when writing stories with pulp and film characters, he brought that sensibility to them. In "The Day King Kong Fell" his narrator theorizes over the sexual drive of the giant ape that lead him to kidnap Ann Darrow and the possibility of his consumating that relationship (the gorilla's penis being quite a bit smaller than a man's, thus proportionately increasing the size may not actually make it out of the realm of possiblity) which might explain why the erstwhile hero John Driscoll dropped her like a hot banana later according to the story (not too dissimilar to Alan Moore's account of the Dracula, Myna and Harker relationship and why she is no longer married at the start of The League of Extraordinary Gentlemen). In A Feast Unknown, he sends up the pulp characters, the violence of the pulps, as well as the overt sexuality in some and the repressed denial of sexuality in others. The end result is a book featuring avatars of Tarzan and Doc Savage, afflicting them with a sexual-psychotic disorder (unable to get erections except through acts of violence) which is culminated by the "heroes" fighting nude with full erections and a battle decided when one castrates the other. When he got the chance to write an official Doc Savage novel, Escape from Loki, he does not resist the chance to deflower the hero.
It is interesting that while he felt the need to explore such areas with the more "normal" characters, he felt the need to malign other characters that already exhibited more passion and had more outre adventures. Through his various biographies and his Holmes-Watson pastiche The Aventure of the Peerless Peer, he tied the characters of G-8 and the Spider and the Shadow together, by asserting that G-8 was delusional and had a breakdown after the war becoming BOTH heroes. Because he was delusional, many of the adventures he was reputed as to having during WWI could be just in his head.
It's not too far of a leap to see how modern comics and creators seem to approach characters with that same mindset. Alan Moore's work often concentrates on sexuality and a sub-theme seems to be supporting that all sexual relationships are normal. Howard Chaykin often brings fetish elements to his works, his mini Twilight is a look at DC's sci-fi characters with the same revisionist sensibilities that Farmer explored. Lately, it has been increasingly popular for such themes and sensibilities to show up in all books as opposed to the fringe books. Identity Crisis brought in rape, madness and graphic murder for a book starring characters used to sell stickers, underwear, and cereal to kids. Black Canary dresses like a hooker for a cussing Batman in All-Star Batman and Robin. Farmer already did it guys. We've had our giggles, let's move on.