I have been thinking of this type of campaign for a few weeks. It's basically an earthbound campaign set in an alternate timeline. I decided to mix and match the politics in Code Geass and Trinity Blood. It is currently trying to expand its territory. Here's a list of the major powers:
The Britannian Empire is composed of the Americas and Oceania. It has recently conquered Japan, although pockets of resistance remain. Note that the UK is not part of the Empire, as it had been conquered by Napoleon in this timeline. The British Royal Family fled to America (the revolution was unsuccessful) and continued their dynasty there.
The Chinese Federation rules much of Asia. Russia may be included in the Federation; I'm not sure if I'll include it here or in the League of Nations (see below). It is at odds with Britannia, although the relationship is a cold war rather than a shooting war at this time.
The Vatican has ruled Europe since Napoleon was overthrown. The Church is ruled by a family who elects one of their members as Pope.
The Ottoman Empire rules the Middle East. Its relationship with the Vatican is cordial, if a bit cool. It is suspicious of Britannia, however.
The League of Nations is a loose confederation of the countries that do not fall under any of the major powers. The League was formed to help the nations preserve their freedom against the major powers.
There are no supernatural elements in this campaign. There are no vampires or beings with Geass powers.
However, there is a lot of high tech equipment available. There are mechas and airships that resemble aircraft carriers. There are also androids and holograms. I may include a few people with android bodies a la Ghost in the Shell, but they would be rare at this time. Weapons are still ballistic in nature, but will have been a few advances, such as Gauss guns.
The PCs can take part in the political intrigue, or they can pilot mecha and enter the field of battle. The campaign can be played from either angle.