I like Inez's idea, hows about two groups of 'heroes' working for different countries, both believing that their country is in the right of any disagreement (Would require fairly patriotic characters.. Or maybe just stubborn ones.)
A good example of that would be like the dwarf vs elf wars.. Neither were evil, they just didn't see eye to eye on things.
I'ld love one where the groups might end up working together, or against each other, depending on individual choices and happenstance before they come to know of each other.
IE if Group A hears that a group of mauraders(Group B) is killing people, then what does it matter whether the rumours are true? Or that these 'mauraders' had good reasons for killing the people they did? And if a member of Group B discovers at the same time that the man who killed his father was saved by Group A, the animosity climbs even higher...
Now these two groups may work through their problems and work together for the greater good (Or evil.)... or they may not.
I'm thinking not going 1 evil/1good would be the best way.. If there were evil members in one or both groups that might work. 2 groups of adventurers working for different people, different groups, wtvr. What one group does may affect the other, and they may even (If circumstances permit) meet one day.
We'll Still have to figure out what each group's going to be, and where they'll be based from, what they'll be doing, who they're working for, etc.
OK, first off it would be best to have 2 DMs, one to handle each group, and both keeping in touch with the other. That makes the load easier on each DM, and if they're not sure of something, they can ask the other one. One would have to be the 'main' DM, so that if there're any inconsistencies they have final say. The other good thing about two DMs is if one has to go away for a while, the other will be able to sub for them until they get back, so the game doesn't lag.
I'm willing to be one of the DM's, and with all the ideas people have thrown around here I think I've got a fairly firm grounding for something here. SO, who's willing to bite the bullet and join me behind the shield to bring this project to fruition?