I’ve thought about this a lot and here’s what I have to say on the matter.
Most fantasy RPGs have 3 main classes, Warrior, Rouge, and Wizard. The class abilities and features of Warrior and Rouge would be easily replicable in the FL universe, but not Wizard, since FL doesn’t have "magic" or spells. Gameplay wise, though, Wizards abilities focus on AOE attacks like Fireball and Cone of Cold, buff/enhancement spells for the party like Featherfall and Fly, and spells inflicting status effects on enemies like Blindness/Deafness and Hold Person. While FL might not be able to replicate the magic of a Wizard, it might be possible to replicate the gameplay effects of a Wizard. Perhaps through an Alchemist, Artificer, or Engineer type class focused on using a lot of the weird items on the FL universe. The Chimney Pot Wars mentions Urchins throwing Primordial Shrieks to stun opponents as some sort of Victorian flashbang. Things liek that could work well in creating a Wizard like class.
A Cleric or Paladin style class could actually work in the FL universe, despite the lack of magic. There could be priests of Stone, Storm, and Salt (and maybe the Drowned Man or Lady Black) and they could place Blessings on their allies and Curses on their enemies. Clerics of Stone would have more healing and buffing "spells". Maybe their holy symbol/artifact would be like the Mountain-Sherd. Clerics of Storm would have more damaging Curses. Since we don’t really know much about salt, i’m not really sure what their Clerics would be able to do.
A Druid or Ranger class could also be possible. Not really focused on nature magic, but on animal companions. Being able to summon or use beings like Rattus Faber, Zee-Bats, Sorrow-Spiders, Frost Moths, talking Cats, Pentecost Apes, Blemmigans?,maybe even Fingerkings or the Eater of Chains. Pretty much anything you can breed in the Labyrinth of Tigers or buy from Nassos Zoologicals.
As much as people would like to have Rubbery Men or Devils as playable races in this completely hypothetical RPG, I don’t think we know enough about them lore-wise to have them be playable. Instead, I think most (but maybe not all) of the races would be human, but would be different based on nationality/origin. Surfacers would be one of the more basic ones, along with Londoner. There could be Presbyterate/Elder Continent people who have a boost to Constitution and, like elves, live for a very long time. Khaganians would probably get a bonus to Stealth and Persuasion/Diplomacy. People from the Chelonate would probably get a strength bonus and maybe could enter a barbarian-like rage/battle fury state. People from Anthe could also be a possibility, though I’m not entirely sure. Iremi could also be another possibility. As for non-human races, I think Unfinished Men, Tomb-Colonists, and maybe even Snuffer could be possible.