The Stand/WMG

(Miniseries-only) Flagg was just a strongly psychic human with a penchant for scheming and a talent for sleight of hand.
The only thing he needed to be physically present for during the Gathering was releasing Lloyd from his cell, everything else was done through dreams and hallucinations. He isn't near as omniscient as he'd like to believe (since he doesn't even notice ), and the shapeshifting into a corvid is done, with only one exception (where he was panicking) in psychic contact. He only turned into a devil to screw with Nadine and increase his fear factor, not because he actually is a demon. And last but not least, while in the book, but in the miniseries,. Mother Abigail's God is probably something close to the real deal, though, if not exactly what she said it was. There's pretty strong evidence that it's not just her, though the possibility does exist that it's just a far more powerful psychic than Flagg (and possibly a precognitive) hanging out in the shadows.
 * Not really...he turns into a crow and flies off.
 * Management?

The superflu is 100% communicable.
Just because it's there doesn't mean you can see it. It has a secondary effect that people with immunity are completely immune to the main part of the plague, but if there's any living tissue after a certain amount of time, it changes fairly quickly into something almost impossible to spot, that may or may not eventually develop into a not-easily-communicable bronchial infection, or a mild 'flu that can be overcome. not because that's how partial immunity works, but because while it was immune to the original 'flu, there was an adapted strain of it that. The reason nobody else got it was that nobody else had it, not that everybody else had "full immunity" to it. It's quite possible that other people have it in various active-but-invisible strains, on top of the dormant but potentially dangerous in the future version.

Somehow, the presence of the Judge in Boulder is part of a Xanatos Gambit by Flagg; since that one man was there, Flagg will be able to return in his current incarnation.
Because the Judge's real name is Richard Farris. R.F. Hmmmmmm...
 * No go. Farris' first name is Teddy/Theodore, not Richard. When he is discussing Hope Cemetery from his youth and he quotes his father, we learn his dad called him Teddy.
 * ...Teddy/Ted is a nickname for Richard. And the Judge's name was definitely Richard.
 * Alternately, he intended to possess or brainwash the judge, and send him back to Boulder to cause chaos,or keep him as a replacement body in case someone managed to defeat him. This is why he wanted him taken alive so badly, and why he reacted like he did.
 * There's also Ray Flowers, who broadcasted just before the outbreak. A disposable avatar of Flagg's, perhaps?

Randall Flagg is a Nephandus.
He's implied to be immortal, can perform acts of rather unpleasant magic that seems geared to destroying things, and is dedicated to destroying the tower, essentially condemning the universe to annihilation. The event that causes the nuke to blow up is paradox backlash finally blowing up in his face when he goes a little too extreme with his magic in front of too many witnesses. Even after his 'death' he appears to reincarnate in the extended addition, lending further credence to the theory.

Randall Flagg is The Slender Man.
He pretty much has the same M.O., until the plague runs its course, and he puts phase two of his plan into action.

This scenario would be a lot different today
The world is way more connected today even than it was in 1991, when the update came out. Everyone with a cell phone has camera and video. And if you're the government, you can't shut down the entire internet and cell phone networks without arousing way more suspicion even than the leaked stuff in the book. Not that I wouldn't put it past them to do it anyway, but then the riots would be way bigger, way faster. And there'd be more leaks whether it was shut down or not, video and photos travel so fast today...one word: YouTube. Or, Facebook. Either/or or both. Viral videos all over the place, people sending photos to their entire email or contact list. Even if everyone got their net and cells cut off, there'd be plenty of stuff out before then. And then probably hackers trying to hack into the system to bring it back up and send the info, stealing satellite signals,to get it on TV, ect. And probably ten million Captain Trips Facebook pages and message boards.

And I often thought there was a chance that might have caused international problems...if either word of the intentional infections got out that way, or the US took the most logical track and blamed the superflu on a terrorist attack and technology let the wrong nation in on it, there might even be mass nuclear chaos before everyone had a chance to die of the plague.

Even without the nuclear part, I would LOVE to see an update of the novel where it's set today, or for someone to write a novel in the vein of The Stand in today's world.

Matt Frewer based his performance as Trashcan Man on the WTTW Max Headroom weirdo.
Seeing as he was Max Headroom, it seems reasonable to assume he would've seen that video at some point prior to doing The Stand, and there are definitely similarities, particularly Trash's Laughing Mad.
 * I stole CBS!

The Stand is set in Carnivale's universe
Flagg is the creature of darkness, and Mother Abigail is the creature of light.
 * And Benjamin is Stu Redman?

Randall Flagg is the Slender Man Mythos
He appears to people similarly, at least until the plague has run its course.