Just an idea I had a while ago.
So it's time for the apocalypse. Heaven and Hell are waging war on Earth, and humanity's cought in the crossfire. The nations of the world decide that they don't want this war, and don't want the world to end. So they fight back.
Heaven would probably be the faction most suited to newcomers to the genre, with powerful units being unlocked fairly early on. As a trade-off, they don't have any uber-powerful units. Hell would start out pretty weak but they get some great powers and debuffs. However, these powers require sacrafices, and are most of Hell's strength. Fortunately, they have a ''lot'' of sacrafices. Earth on the other hand, has TwentyMinutesIntoTheFuture technology. Their campaign ends with them nuking Dis.
Also, both heavenly and infernal forces have upgraded their weapons since biblical times. Earth still has the best stuff, but their tanks can't shoot hellfire.