A turn based war strategy game revolving around a war between heaven and hell, with Earth caught in the middle. The forces of heaven have Archangels, lower order angels, various heavenly prophets who can recruit believers, and such. The advantage of heaven's forces would be in their mobility; angels especially would be able to strike hard and strike fast, crossing dimensions and continents in surgical precise blitzes.
The forces of hell would be asymmetrically different. Where heaven's special units, the angels, are based around individual power and quality, the demons of hell are dissimilar in that they are equally powerful, but are more based around corrupting the landscape and mortals around them. Whereas angels would hit and run hard enough to level nations, demons can alter the very laws of nature around them, twisting everything until the land serves as a bastion and spawning ground for further hellish entities. Camping a location, for a demon, is a smart strategy. Hell would also have a greater assortment of very low tier monsters and oddities, although the logistics of transporting them across oceans would limit their spread, as they can only come from the Mouth of Hell or be spawned in corrupted-enough locations.
Humans, the third playable faction, are fucked in the early game. Capable of battling only the weakest soldiers of heaven and hell, your forces, while very numerous, are helpless against orbital bombardments from angels and having both body and soul defiled by higher order demons. The goal would be survival. Obviously, one strategy would be to just flat out side with one of the other two factions: Side with heaven, and you provide the celestial hosts with recruits and the chance to spawn God-touched heroes, new prophets, and holy warriors in numbers that hell cannot hope to match. Side with hell, and the infernal regiments would be able to send greater demons, even Archdemons to Earth through hosts, as well as provide fresh numbers to hell's overall army.
Staying neutral as humanity is also an option. From there you'd have some interesting options. One would be to somehow survive long enough to develop weapons capable of hurting both sides, which are mutually exclusive, but if successful, allow you to take out the heaviest hitters of both sides with your fully upgraded military forces. Victory through specific anti-angel and anti-demon ordinance and equipment adaptations, all this is possible and feasible given humanity's large numbers, spread of forces and lack of important individual targets when compared to heaven and hell.
Other options ( probably DLC ) would include siding with Eldritch Lovecraftian gods, taking steps to ensure humanity evolves into physically, mentally, spiritually superior ubermensches capable of actually taking on angels and demons in every arena, seeking aid from extraterrestrial aliens, etc.