I am fine with either Tri or Dual color.
Even if the war is not between human and elves (That was just an example anyways), I still think it would be nice to have two or more large factions be over smaller factions. A very complex war. The larger factions can be tri color while the lesser factions like the individual provinces be dual color. or not.
Ok, here's my take on the idea: 5 tri-color factions (most likely shards) are our large factions. Then 5 dual-color factions (if we use shards, we have to use ally colors here) are our lesser factions. We can have them differentiated by creature types, using the race/class set up as normal. The tri-color factions would be our classes, such as grixis (UBR) being a coalition of wizards. The duals would be races, such as RB being goblins. Each race would fit into two of the classes.
I like the idea of the world being in a complex state of war. I'd think it'd be a WWI type scenario, with two of the races getting into a conflict which drags the rest into war due to the class-based alliances.
By the way, we should answer some simple questions to flesh out this world:
What is the terrain make-up of this world?
So, the current suggestions are provinces on a single main continent, and a bunch of factions spread across islands. Of course, I'm in favor of the islands idea, since I don't think there has been such a plane yet. Possibly there was previously a larger continent that sunk to the depths. Possibly the factions rose up from the remnants of an ancient civilization who's entire legacy rests at the bottom of the ocean?
What is the level of technology on this world?
I'm a sucker for low-tech tribal scenarios, but I feel like that wont fly here, so I'll leave this to you guys.
What creatures inhabit this world?
Does this world rely more heavily on certain colors of mana (such as innistrad being inherently more black)?
Are we taking inspiration from any real-world sources?
Ally or enemy colors?
Who's at war, and for what reason?
And most importantly, what is this plane's name?