Hmm, if I was rewriting the story to suit what I find interesting?
I wouldn't make everything awful being a part of some demon-armored villain figure. I would probably instead focus on just being in the middle of a warzone and dealing with how each faction are both perpetrators and victims of the war that has already begun.
At it's heart, vikings and knights having to deal with each other can be squeezed to make sense. The knights are the economically powerful faction who have an expansive empire, while the vikings are on the fringes of that empire, are of much lower economic power and so make ends meat by raiding them. Throw in some century old grudges where King Lothair captured and beheaded the great raider Uthrum or something just to get a bit of background and pathos to the characters.
The samurai faction being a forced migratory group, possibly running from something sinister back home could still work. Just that means they have a bunch of tensions with the two previous factions: They need their own land which is currently being held by the knights, but maybe it once was territory owned by the vikings and lost during the knights expansion a hundred years ago or something. The samurai would need to acquire land and defend themselves from the other two factions, forcing more conflict.
And speaking of characters, I wouldn't call the player character "The Raider" or "The Warden" which just seems lazy. If I had my way, each campaign (there would be one for each faction of course) would center around 4 main characters that are actually fleshed out characters and not just titles. Each of these characters could showcase a view on the war. For example, the Warden may be the king's personal bodyguard, giving a political view that they cannot stand by and let their lands be taken away. While maybe the Peacekeeper represents a more common person who just wants the violence to stop, maybe even sees the samurai as a potential ally against the raiders that no one thinks can be reasoned with. And each mission would feature one of these heroes trying to go about carrying out their objectives, creating alliances, and at times failing to get the concessions they want.
If you have a good writer with a nice bit of twists and turns, the game would culminate in an all out battle where the player gets to choose which of all the heroes they wish to play, and that decision determines not only which of the factions wins, but also what happens with that win. Like, if to use my previous example of the Warden and the Peacekeeper. If you choose the Warden, the vikings are expelled from the lands, while the samurai are pushed to the very corner of the map. While if you choose the Warden, the knights still win, but at the end maybe they use that victory to show some mercy to the samurais and allow them to have their current holdings but for some concessions.
And then, of course, we set ourselves up for a sequel, with maybe that force the samurai were running from show up (to simply use my historical parallelism as an example I'd pick the Mongols, I'm just not sure how to do them effectively without cavalry, really any new faction could work here).
Anyway, that's how I could see the story being improved, as someone who, you know, hasn't ever played through the campaign. Just from what I've seen the focus seems to be an evil overlord, and the missions we saw had people just calling your player by his or her class which just seems so suspension of disbelief breaking.
3 people found this helpful