Just for the sake of pattern completion, what do we think about enemy-colored takes on the Unstable world? I don't have any card designs atm, but feel free to add them if you like.
WB - Parody tech company, enforcing sameness to maximize their market share, maybe some sort of mechanical theme about overwriting types or p/t or colors or whatever and taking advantage of that. Black and white both have a minor history of imposing their own colors.
UR - This is the hardest one since the whole premise is basically Izzet already, and almost any of the factions could have worked in these colors. Only thing I can think of is a parody Izzet group that is like a stupid dragon served by a bunch of smart wizards or something. Or I guess you could just literally have a group of Izzet who ended up in this world though some experiment they did.
BG - Some sort of junkyard collective, similar to the sentient trash computer thing they build in Perdido Street Station by China Mieville. I think there are lots of Un-ways to take advantage of the graveyard that haven't been explored yet, like seeding cards in opponent's graveyards or mini-games involving graveyard cards.
RW - At first I thought Dwarf military engineers, but that might be too close to the Kaladesh dwarfs? Santa's Workshop toy-building gnomes is another idea that fits the literal color scheme and could explore strong positive emotions as a red/white we don't usually get to see. Mechanical theme of donating permanents (like Bronze Bombshell), exchanging control and so on.
GU - Unfortunately they kind of used the best idea here for GW. I guess plant-based contraptions are the obvious route here?
Love white-black faux-Apple and black-green trashpicking hoboes.
Blue-red: Disney/Willy Wonka/X-men villain Arcade - Makers of secretly deadly items themed around fun and leisure, headquartered in a huge theme park.
Red-white: Not just Dwarves, but Gnomes and Kithkin; leaning hard into the Germanic stereotypes by being jolly makers of toys, intricate clocks, and other such things while also being carousing drunks who want everyone to party equally hard.
Green-blue: Damn dirty hippies with "psychedelic" Contraptions, a la Yellow Submarine.
Great ideas! Here are my thoughts, some are pretty similar to yours already.
WB - Greed company who try to monopolize the market and make everyone... except themselves, of course, equal. Makes something like servo tokens and makes your opponents sacrifice or tax things that aren't inline with their product.
RU - the easiest imo because these colors are Izzet and they care about artifacts and experiments that do wild things... but that's too easy. We could follow this same trend of mad scientists, or if we want we could go with airship pirates and explorers? I think an Izzet feel but focusing more on time magic could be cool as well. Red is all about speeding up time and blue is all about slowing it down... or any whacky things the two could do combined!
BG - Another easy one. I like the massive scrapyard idea involving lots of Golgari like things but with a bit more machine based technology involved. I like a recycling and scrapyard theme for them. I think of cards like Scrap Trawler. Feels like Dredge could make a comeback here on a whacky card, kind of like Storm on Crow Storm!
WR - Stealing a bit from Kaladesh, I like the idea of heroic vehicle riders running around on contraptions in the streets. Fast and Furious. Maybe even some advanced weapon builders... Maybe they make giant mech suits like Titanfall and brawl it out like gladiators? Yeah!
GU - As said before, crossbreed labs kind of stole the thunder from simic. But I think another idea here would be counters and the manipulation of counters to do really weird things. I was thinking maybe like a virus or a germ type of science, where you can manipulate, grow, and transform things by using contraptions to fuel your germ!
Tomorrow I'll post some custom card designs for each of my proposed factions to see how they feel.