I mean, for crossover universes my immediate instinct is Lord of the Rings. But honestly, it almost feels like that would be redundant cause Third Age: Total War exists and is frankly one of the best LOTR gaming experiences out there period. Seriously, if you have Medieval 2: Total War, check out Third Age. It's pretty freaking awesome.
Historical settings, there's the Three Kingdoms period of China which could be really cool. World War I or World War 2 *could* be cool, but require a LOT of new mechanics and adjustments to account for how wars were fought by that time, including aerial combat and use of bombing and such.







