The alternate universe where AOE3 was released with historical campaigns. The formula is going to be very similar to AOE2 ones. If we are to have real campaign additions later this could also offer a reference.
Disclaimer: This involves all 14 vanilla, TWC and TAD civs. I’m not separating out the expansion civs because I also think the narrow regional focus at release was not a good idea (e.g. it could have started with 4-6 European including Ottomans, 2 Asian and 2 native American) How the civs are distributed across expansions is a different question.
Napoleon
From the French revolution to an alternate history Waterloo where French won, but the future was still unknown (just like the end of the AOE2 Montezuma campaign).
Peter the Great
Swedes may not have been in the game, but fusiliers and hakkapelitts already are. So it’s easy to make a campaign-specific opponent.
Koxinga
Koxinga’s campaign in Southern Ming and Tawain.
Historical battles
Count as a campaign. Many of the DE ones could be included. Also some other options:
Toba-Fushimi (Boshin War)
Lutzen (Thirty Years War)
Marston Moor (English Civil War)
Waterloo (British perspective)
Malta (why not)
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More to be added in the expansions:
American expansion
American Revolution (a more historical Fire campaign)
Black Hills War (a more historical Shadow campaign)
Beaver Wars (Iroquois and Huron’s struggle over fur trades)
Asian expansion
Sepoy Revolt (a more historical India campaign)
Ismail I (this should be an AOE3 campaign!)
Tokugawa (a more historical Japan campaign)
African expansion
Kassa Hailu (extended Ethiopian scenario)
Fulani Jihad (extended Hausa scenario)
Hayreddin Barbarossa (extended Algiers scenario)
European Revolution expansion
William the Silent (Dutch)
Garibaldi (Italian)
Greek Revolution (as the name suggests)
And more historical battles depending on the civs added.