DCC Thundarr game, I wrote about post-apocalypse archetypes. In this post I'm adapting the DCC core classes. Guns are more common in this setting so all classes are trained in firearms
The warrior needs little adaptation. A warrior can perform Mighty Deeds with firearms: shooting a weapon out of an enemy's hand, shooting them in the kneecap, shooting near a target to distract them, etc.
In the world of Thundarr, Wizards are known as Cyber-sorcerers. Their magic is a result of Super Science! Each wizard has a cyber implant that is the source of their magic. For example, a wizard might have a cybernetic eye. Magic Missile would be a laser that fired from the eye. The eye could scan an item to perform Detect Magic. Patrons alter those that bond with them. The patron may infest the Wizard with nanites or implant a device in their brain. Spellburn and corruption often result in the wizard becoming more machine-like. (Look for a future entry on PA spellburn and corruption!)
+Tim Callahan's excellent Technomancer class in Crawljammer #2 expands this idea to an entire class.
Thieves become Scavengers, digging through ancient ruins (and other people's stuff) looking for artifacts of the lost age. Scavs may backstab with a firearm if the target in unaware. Silent weapons are a Scavs best friend. Pick Lock, Find Trap, and Disable Trap work on their hi-tech equivalents. Forge Document can create imitation keycards and ID badges to access strongholds of the ancients. A Scav can use the Read Spell from Scroll to use arcane cyber-sorcery from ancient machines.
Fantasy-style gods aren't a good fit for a post apocalypse setting. So where do Clerics get their abilities? Here are three different ideas.
Techno-Cleric (Law): The cleric's abilities come from Techno-devices. Lay on Hands is a med-kit. Turn Unholy is an emitter that drives away certain kinds of creatures. Each spell can be its own item or a combined item. A special environment suit might contain all the cleric's abilities. Disapproval functions as normal, representing the Techno-Clerics need to repair and maintain their gear. Lawful clerics are usually part of an organization working to restore civilization. (See my Archetypes post for some examples.)
Cultist: (Neutral): The Cultists abilities come from studying the secrets of the Great Old Ones. They are uncaring of their followers. Disapproval means the Cultist has attracted their attention.
Cyber-Priest (Chaos): Cyber-priests are altered by powerful patrons to be their servants. The Cyber-Priests abilities come from nanites, brain implants, genetic modifications, and mutations that their patron has bestowed on them. The Cyber-Hive from issues Two and Four of Crawling Under a Broken Moon is a great example a Cyber-Priest patron. A Cyber-Priest may automatically choose Patron Bond and Invoke Patron as part of their starting spells.
Halflings are a new race of tree dwelling mutants called Minks. Part monkey and part squirrel, they have prehensile tails and live by scavenging among the ruins. Minks employ their prehensile tail when two-weapon fighting.
Elves are a unique race known as Sorcerers. Sorcerers appear human but are either a new
race of mutants, aliens from another planet, or dimensional travelers. They employ magic without the need to resort to demonic pacts or super science. Sorcerers never bond patrons or spell burn. As a result, they never suffer from corruption. A Sorcerer may choose to spell burn or bond with a patron. This removes their immunity to corruption.
The cataclysm that destroyed the world didn't happen overnight. During the fall, great underground vaults were built. Many survivors retreated below ground. After a thousand years, these "Vault Dwellers" bear little resemblance to their human ancestors. Vault Dwellers can detect hidden caches of technology. Vault Dwellers have a tradition of self defense. They replace sword and board with martial arts. The Vault Dweller may make an martial arts attack with their off head using the rules for shield bashes. They may also choose to forgo the extra attack and instead dodge incoming attacks. This adds their deed die to their AC for one round.