In terms of "things that will surprise absolutely nobody that's heard me talk about ARGs for 15 minutes," I've had one grand ARG idea rattling in my head for ages. It's actually tortured me for years, because I knew that it'd take an act of divine provenance in order to make this theoretical ARG. So I'll share some of the basics with you, in part to get it off my chest, and in part because I bet at least one person here will be tortured by how awesome it could be.
It'd be called We Are EarthBound, and it'd be a crossover with the world of the video game Mother (the second of which was released in the US as EarthBound). If nothing else, the fact that someone earlier this year launched one called We Are The Earthborne means I'd need a new name.
But the idea would be that, due to the temporal holes made by Giygas, in addition to the effects of various characters' psychic powers (particularly Ness' powers, which did leap across the fourth wall and contacted players), echoes of these characters were starting to appear online. Most confusingly, several characters turn out to be temporally displaced versions of each other, including a big reveal that two characters antagonistic to each other were BOTH Porky Minch, displaced from different points in the Mother timeline.
The two parts that I had a strongest mental vision of were one of the rabbit holes, and one of the climax interactions.
I had the mental image of using magazine ads for rabbit holes, particularly if they were going to be promoting a DS rerelease of the Mother trilogy (I mentioned "act of provenance," right?). There'd be two primary phases. One would be silhouettes of various player characters black on a red background, revealing aspects of their character with sentences starting "I am..." with the last sentences being "I am ready. We Are EarthBound." The second phase would be red silhouettes on a black background of the major villains, with them doing a similar reveal, except it ends with "I am coming. We Are EarthBound." Best would be the Mother 2 version of that, because it'd feature the "howling abyss Giygas" picture, and *only* the consistent villain lines written, in a font reminiscent of scratched-out wood. I still love the mental image of that, because the idea would be the on-page equivalent of unnatural screaming of that by Mr. "You Cannot Grasp The True Form," which would have given folks nightmares.
Speaking of, one of the center pieces would be a puzzle that players would solve. It'd serve the purposes of some characters, even some good ones, but there'd be once character that was nice that slowly became more unhinged the more that puzzle was solved. Turns out that this character is a version of Giygas from between the first two games that had desperately given itself Laser-Guided Amnesia to prevent from being driven mad based on the emotions that Queen Anne's Lullaby awoken in him. Completing the puzzle would result in the release of an audio file where Giygas finally regains his memories... and you have to listen as the character that you had grown to know is quickly consumed by his own madness to become an Eldritch Abomination.
As much as I'd love this, this would involve three things that are almost certainly never happening. 1) Me, being hired by Nintendo. 2) Nintendo deciding that a Mother anthology needs to be made and released in the United States. 3) Nintendo suddenly believing that an ARG would be a great idea (they've made fun of them in the past).
As a side note, folks may notice some parallels to what I outlined above and what actually happened in TWWF. Some of that is pure coincidence, and some of that is actually due to me suggesting ideas that originally came to me in the mental development for We Are EarthBound for use in The Wall Will Fall. And I'll admit to one more thing - there are several ideas that I'm not discussing right now, because I could easily see some of the mechanics and plot ideas I came up for WEAB could be used in a sequel to TWWF.