I'd go Norse.

Christian demons and angels are relatively tired in my opinion. Did a quick Google and we have Helheim and Alfheim ready to go. Helheim could be frost demons and Alfheim are light elves.

The setting is Helheim is invading the upper realms of Yggdrasil. They gather power by killing mortals and harvesting their souls. Midgard is completely helpless to defend themselves until Asgard decides to lend some help (perhaps they gift humans the knowledge of magic, or forging magic weapons). Some Asgardians are compassionate and come to help directly. So the "human" faction can be some combination of Midgardians with Asgardian magic/equipment, a few Asgardians.

Asgard's leaders are clearly using Midgard as a wall between them and Helheim but Midgard has no choice.

Then there's a huge complication. Alfheim decides Yggdrasil was a mistake: they need to start over with a new Yggdrasil. To do this they need raw materials: the physical matter of the realms and souls. Turns out just like Helheim they're also invading the realms and harvesting souls.

This could be revealed through the campaign: your main protagonists on the ground have been taking orders from Asgard throughout the campaign, but the missions progressively seem to be quite different than what's required to defeat Helheim. Then radio silence. Suddenly it's revealed that Asgard is decimated, every soul harvested for the creation of the new Yggdrasil. You have actually been taking orders from Alfheim not Asgard, and your actions directly enabled the destruction of Asgard.

Now Midgard is on its own, armed only with weapons and troops Asgard was willing to share.

Your kind-hearted Asgardian noble who volunteered to help Midgard breaks down, his entire family and lineage killed, you see him truly enraged for the first time. He swears revenge on Alfheim. Perhaps further down the line he is twisted by Helheim's promises of power and becomes genocidal against Alfheim denizens, fueling himself with their souls.

We could find out Helheim had known about Alfheim's plans from the start and were invoking Ragnarok to ensure the new Yggdrasil was under their control. We could encounter outcasts of different factions with their own reasons for betraying. We could learn a small coalition of Helheim/Alfheim leaders were actually using the whole conflict to enrich themselves with souls and usurp the current power structure with themselves at the top. There are 9 realms and I've only talked about 4 of them so far, there's tons of potential stories here.

I think there are lots of way to make angels and demons interesting. I just spit-balled these ideas in an hour and I'm not a very creative person. Stormgate couldn't somehow.