Okay, state of modern magic, everything is on the table. I was mulling over things like 40k and Trench Crusade and had a thought of a plane meaning to evoke a sort of "World Wars" vibe. Machine guns, barbed wire, gas masks, trenches, and a very grim meat grinder existence. The war would have been going on for longer than either real world war, but for consistent worldbuilding probably not to the "forever war" sort of trope. Maybe it's been going on for about 20 years and is in what seem like the last days before everything breaks when we visit, so that the kids in the trenches have never known peace.

It would still have to be Magic, so fantasy elements would be used to help add to the look and feel of a long, grindy, machine-age war. Like instead of planes, we'd have dragons filling the role of bombers and drakes or wyverns as fighters. Groups associated with could use necromancy to conscript their fallen soldiers back into service time and time again.

I had the thought that each color would sort of be associated with aspects of warfare. The countries involved in the fighting would have very broad identities -- I hit on the concept of them being 4c factions, since while national identities and circumstances could skew their experiences, they'd largely be trying to do everything as nation-states and wouldn't have incapacity in too many realms.

would be strongly associated with the folks in the trenches. The common soldiers, fighting for their land. Dug in, defensive
would have naval power and to an extent airspace control
would have chemical warfare and necromancy
would be about scorched earth and bomber-dragons in the air. Wizard artillery.
would deal in guerrilla fighters and supply lines.

Your two color draft archetypes would look something like

- Artifacts/Vehicles
- Gravedigger
- Morbid/Revolt
- Raid
- Food
- Soldier Tribal
- Batallion
- Fliers
- Sea Monsters/Go Tall
- Aristocrats

As for the nations

: Sort of the England analog. A naval power that was among the first to industrialize. Their lack of green suggests that they might have strained supply lines
: Sort of the French analog. Dug in and fully in turtle mode. These are the guys who are going to both love static defenses and necromantic "recycling"
: Maybe a nordic-feeling nation? Possibly majority nonhuman, likely elvish. Probably pulled into the main continental conflict more recently, and are struggling in the face of the realities of war
: These would probably be the quasi-germans, at least in terms of their material position. Landlocked, fierce fighters, determined to push the attack while fighting on every front
: Without white I feel like this faction wouldn't really be invested in the homes and lands. At first I was seeing this as a faction of mercenaries, but more simply they might be the American analog -- late to the fight and/or playing an away game. Could be both. A foreign power selling elite troops to the highest bidders.

I think it would be important for evoking a feel that's more of the WWI story than the WWII story that the war itself would be the great tragedy and "enemy" more than any particular kingdom being "the baddies". If you wanted to tell it like the sequel you'd get more of a good-versus-evil vibe but I think it would have more potential (especially since players should want to play any faction) if it was just a bunch of flawed mortals