Although I've never played in the Eberron setting, I do have a campaign that allowed alot of GvG fighting and roleplaying. I understand that Eberron has alot of guilds and factions, so theses could be substituted in for the ones I made up for my campaign.
Basically the heroes start off in a tavern in a small town on the far reaches of their country. For whatever reason, they are all in need of some cash so will be looking out for jobs. At this point in time the kingdom is peaceful, tranquil, basically the 'perfect world scenario'.
Anyways, the party gets a mission from a local church (preferably one of the larger good churches, it helps if there is a cleric of that church in the party) to deliver a package to the Capital. This package is of vital importance.... However, the package needs to be there in 3 days time, but the Capital is 2000 miles away. The party gets directed to a portal to the Plane of Shadows, which will distort time and space allowing them to exit at an ancient portal near the Capital.
So the party goes through and exits near the portal 3 days later....but theres a problem: Even though they have been gone for only 3 days, when they exit the portal 3 years have passed. The kingdom is now under the control of a warlord, martial law has been instated in all the cities, taxes are up, the usual. And to make matters worse, the current warlord was elected fair and square by the populace, as he promised to protect them from an even greater evil. So in other words, the good alligned town guards, paladins and even some of the general public are the enemy.
At this point the party decided to start a revolution within the Capital by getting various faction to join an Uprising. Obviously, you can attach aditional quests to these. The factions I used were: The Guild of Shadow (Joint Assassins & Thieves Guild), Dockworkers Guild, War Veterans, Gnomes, Druids and of course the Churches.
From here you can expand the campaign, either letting them kill the Warlord and letting peace return. Or after the Warlord is overthrown...the greater evil does in fact strike the kingdom...