given the fey exist in timeline 300, perhaps they should go a bit Artemis Fowl? Triffids might well be their creation (wood elves meet mad science!) I'd note in the origional book Triffids were a product of the Soviets.
http://en.wikipedia.org/wiki/Allies_of_World_War_ISuggests Russia(pre-revolution), Japan, USA, Benelux, Italy, Siam, Greece, Liberia, Canada, Australia, Nepal, India, Serbia, Montenegro, Romanina, Brazil, Portugal on the Allies side
Austro-Hungarians, Ottomans, Kingdom of Bulgaria for the central powers.
That's a great mix of stuff to pick and choose from.
Japan = steamunk gundam. no question. (and steampower rangers?)
USA. It's a bit cliche to have it as a kingdom and have the confederacy win. Let's make it more interesting:
Hmm. the US Civil war was sparked by one man's invention of the cotton gin, making the slave cotton plantations economically viable again (so the south didn't want to give up slavery). His later innovations in mass production helped the North to win the war.
What if, given the tech level in 300?, he'd invented a working cotton harvester?
http://en.wikipedia.org/wiki/Cotton_pickerWithout needing the huge labor force for picking, slavery wasn't economically worth it. With the economic imperative behind them, Southern plantations industrialize as quickly as the north does but tension grows over different tariffs set by a southern democrat controlled Congress. The working black population (and urban poor everywhere) are subtly encouraged and influenced by German Marxist agents. The WWWI Russian Revolution happens in America, with Fascist groups like the Klan, the Southern Cross and the Knights of the White Camelia, fighting Commies who are fighting democrats, who are fighting repbulicans. This quickly stabilizes into Doughboys vs Dogfaces, (roughly isolationist, strong states vs intervenstionalist, strong federal gov) Like the Red vs White war in Russia, there's no convient geographic lines, with he same area being fought over many times.
Tsarist Russia then becomes a very backwards mass peasent army, with very strong mystical elements and mongol bikers. Anything to get away from the 'really big tanks' stereotype.
As for more unusual factions - how about the Astro-Hungarians or the Astrolasians?
(former british empire penal colony on the moon. Rebelled and is now performing lightning raids on scarce resources.)