Well, one way to work with both high adventure themes and global threats? Do it Lord of the Rings style.

Party forms for adventure > is threatened by agents of Big Bad > ends up on a quest to stop the Big Bad > eventually there are big armies and stuff and oh God is that a giant elephant?

This would let the roleplay have both high adventure themes and, y'know, war. It would focus on individuals and their quest. Battles and such would be a backdrop, almost.

Also, I think it would be good if people were to keep their minds open as to what the result of the plot could be. Evil could lose, or the perceived evil could end up freeing itself from the actual evil (THE HEART, maybe), or the perceived evil could win and thus lead to a different sort of story altogether (one of resistance, for example). Smaller things should be left open, too. Player characters can die. Nations can crumble. Etc, etc.

Sorry if this is a rambling post. But yeah, basically: the rough idea of the plot should be known, but the exact outcomes and the road taken there... not so much.