Sealed away in a demiplane? Check. Rovagug and Tharizdun are aspects of the same god worshiped on Golarion and Oerth, respectively. Myths about them have diverged wildly over the ages, leading to many differences in the details, but the core story is the same. Other possible aspects include Khyber from Eberron, the wraithlike Dark God (see 2E's Monster Mythology), the oozing Elder Elemental Eye (worshiped in Greyhawk), and the Drow god Ghaunadaur.
Aroden knew his death was coming.Aroden had fostered two goddesses risen from mortal humanity, Aranzi and Iomedae, because he knew he would need a successor. Either something was coming for him, or some inevitable threat was on its way that would require a Heroic Sacrifice from Aroden, the last Azlanti on the planet, himself.
What Zon-Kuthon did to his father wasn't malicious.We know that ZK still feels in his way some of the attachments he had during his days as Dou-Bral (see: Shelyn), and that he has a severely warped sense of what is and is not desirable. Maybe he just wanted a little Nightmare Fetishist father/son bonding?
That unspecified otherworldly wrongness that turned Dou-Bral into Zon-Kuthon? The internet.To be specific, direct psychic contact with the more disturbed and disturbing corners of it. Encountering either some of the weirder manifestations of the Furry Fandom or absorbing the psychic energy of the Furry Hatedom is probably to blame for his attack on his dad. (Who, remember, was a wolf-spirit.)
Lamashtu's influence has reached Earth, and Lady Gaga is her herald.Lamashtu is the three-eyed Mother of Monsters and associated with childbirth and fertility. "Born This Way" is "the manifesto of Mother Monster", features lots of weird birthing imagery, and shows Gaga with an extra eye in the opening monologue. Consider also that Lamashtu is the goddess of madness and nightmares, and Gaga's videos are known for their generous doses of creepiness and WTFery.
The SCP Foundation has at least one minor Spawn of Rovagug in custody.682, clearly some kin to the Tarrasque, is the obvious one.
Imrijka is actually a Jägermonster.She definitly would not look out of place if depicted in Girl Genius: She has green skin and fangs and wears clothes that fit perfectly, most importantly the hat. She probably was somehow teleported to Golarion- which could be both thanks to a Spark's experiment or due to conjuration, and has false memories according to which she is a half-orc.
Aroden fell into a black hole.That's why his death couldn't be foretold: because the "no information can escape" rule applies to metaphysical information being flung backwards in time (the raw material of prophecy) as well as mundane stuff. (Not to mention all the temporal weirdness inherent in black holes and such.)
Alchemists' Sunlight Bombs are tiny hydrogen bombs.I suppose it's more likely that they just include some kind of flash powder that gives off ultraviolet light as it burns, but - Rule of Cool.
Nethys is really Nyarlathotep.We've been told in many places that Narly had a great deal of influence over ancient Osirion, but in "Lost Kingdoms" it is revealed that Nethys manipulated the first Pharaoh and his successors to create and shape the kingdom. Perhaps the insane god of magic, creation, and destruction is really the Dark Pharaoh in a guise that lets him be worshiped alongside the more conventional deities without raising an alarm?
Aroden faked his deathWhy? Who knows.
Aroden killed himselfWhy? Just before his scheduled return, he saw a prophecy. Something bad on the 'Rovagug gets out" scale. The only way to disrupt it required a major sacrifice. Something akin to the life of a God.
The 4th Bestiary will not stat CthulhuThe cover of Pathfinder Bestiary 4 appears to show the great old one Cthulhu. Let's think about this, Chtulhu is essentially a God and in Pathfinder Gods stas usally look something like this: CR: Ha Ha Ha No. So what's the creature shown on the cover? This tropers theory: Star-Spawn. They've been statted before, just not in one of the bestiaries and they've been depicted as being beatable (albeit very difficult to kill).
There will be an adventure path that visits EarthCthulhu is supposed to be slumbering on a distant world - obviously Earth. So there's an opportunity to use the Cthulhu Mythos as a bridge for Pathfinder characters to visit Earth. However, unless Paizo wants to get ambitious and use this as a launch for a Pathfinder Modern game, they probably won't visit modern Earth. The simplest solution would be to drop the characters somewhere in Earth's past. My personal hope would be for the Thirty Years' War - a little more advanced than Pathfinder (ubiquitous guns), full of religious intolerance, and generally not a fun place to be.