Savage - The baseline fighting class, suitable for Thundarrs, Conans and any uncivilized human who gets by on force of arms. Perhaps not much different from a standard Labyrinth Lord fighter.
Scavenger - The uncivilized human who gets by on stealth and guile. I.e. an alternate thief of some sort.
Techno - A human knowledgeable in ancient tech and science. This class would be reserved for unfrozen people from the Science Age, refugees from Logan's Run style arcologies, time travellers, visitors from advanced extraterrestrial colonies that survived the Fall, etc. Inspired by the Arduin class of the same name, though I'd be more inclined to use the St. Andre version.
Sorcerer - The basic human arcane caster class. I'd like to stick as close to the standard MU as possible, but I also kinda want to ditch spell books for a know it/throw it approach. I think I'd like the default magic-using PCs to be just as illiterate as everyone else in the party.
Wizard - I can't run a Thundarr game without giving the players the option of running one of those awesome mutant techno-wizard guys that Thundarr is always fighting. This class would combine the skills of the techno and sorcerer with the possibility of mutating into something horrible. Mechanics-wise, it would be akin to the Elf class: lots of powers but a painful XP chart to climb.
Witch - An arcane caster who gains additional power by making pacts with extradimensional demons.
Prophet - The cleric class of the setting. Instead of serving a temple and a distant god, these people directly hear their god's voice. Think Joan of Arc in the year 4094 AD. I may drop the ability to affect undead or make Turn Undead into a spell.
Cyborg and/or Robot - Because cyborgs and robots are cool.
Deviant - My version of a mutant class, with name inspired by the Deviants of Marvel Comics. Instead of mutant powers that level up as the character does (the Mutant class in the back of MF) I would go with full-blown MF uberpowers. The catch is that the character starts with very few mutations and continues to mutate in play.
Brute - Because if someone wants to play Ookla the Mok I sure as hell ain't gonna say no. I might borrow the Half-Ogre class from Ruins & Ronin for this one.
That's ten classes, which feels like too long a list to me. I guess the Wizard and Witch could be folded into the Sorcerer as subclasses, bringing the total down to eight.