In my mind's eye, I envision this to be sort of tournament-style play where multiple groups play each round and outcomes get reported back to a "master judge" who can then give results, story cues, and other details to the GMs to run the next round. So each round should build on the previous.
Players would likely generate a few different characters at the outset, namely a few pure-strain humans, several mutants, and a few alien characters. At least three of these adventures (currently, anyway) have alternate takes where players could play for the other side (I've marked them as ALT).
ROUND 1: MUTANT SNIPE HUNT
You're a member of an elite team of pure strain humans called "Valids" that is out to clear the surface world of mutant scum and make the Earth safe or habitation again. Your team is one of several culling squads sent to hunt, capture, and eradicate the mutated hordes.
The hunting parties are given profiles of which mutants are valued for scientific study (capture) and which are too dangerous to roam (kill). Take your crackerjack team of specialists to the surface in a mechanized drill tank along with your Mother May-bot (artificially intelligent chaperone/drill sergeant) to keep you in contact with other squads and central command.
Good luck and Mother's Blessings be upon you!
ALT: Mutant players must rescue their comrades from a the brig inside the giant drill tank before it dives back beneath the surface.
ROUND 2: ARENA ROCKED
You're a mutated survivor of the Great Calamity who's recently been captured by those pompous human under-dwellers. They tossed you into a holding area with a great many other mutants where you were shipped to a walled-in city. There's little food, but LOTS of weapon caches in this abandoned urban wasteland. It's been turned into some sort of gladiatorial arena and you'll have to fight your fellow mutants to survive. But wait--there's an active resistance growing! It's time to set aside petty differences and form a fighting force to overcome your oppressors and escape!
ROUND 3: WALKING DREAD
You're an outlander--a human or mutant wanderer of the Gammascape's furthest reaches. As you enter a small city strange happenings give you an uneasy feeling that things are about to change in a big way. You stumble upon a skirmish between pure strains and mutants. Before you know it, a horde of zombies arrive to feast on the survivors. Outnumbered and overwhelmed, now seems like a realy good time to stick together--humans and mutants alike!
ROUND 4: INCOMING TRANSGRESSION
You're part of a new breed of Gamma Terran--an allied band of humans and mutants, trying to restore order to the ruined chaos. Reports keep streaming in about weird weather and the reanimated dead. You've got to fight off the necrotic hordes--and make allies of former foes--before something even more terrifying from beyond arrives to take control!
ALT: Undead players must divide and conquer the humans and mutants. Feast alone won't do it--they've got to turn some of them in to increase their own numbers in order to overwhelm and claim victory.
ROUND 5: BATTLE GLOBALE
The invasion has begun! Alien aggressors want this planet--ruined or not--for themselves. Only you and your coalition of humans and mutants stands in the way. Make your way to the World Security Council's HQ and activate the World Alert Beacon--the last remaining scrap of global civilization that could be humanity's only chance to call all Earthlings to arms. It's an all-out, gamma-lit, firefight. Who will claim victory in the Gamma World War?!
ALT: Alien characters are trying to put down the Earth's remaining humans and mutants and stake their claim in taking the planet!
Five adventures (modules?) seems like a good number, being not too long but still hitting all the key plot points. Plus, there's still room for a robot uprising, weaponized kaiju, and cyborg raiding party or five that I'd like to sprinkle in. :)