I know CA must get loads of suggestions for new total war games, but I think the great northern war would be a great setting. You have a nation with a foothold in Russia, Poland, Denmark, and Norway. It has war declared on it by all these nations in 1700, only shortly after an underage king is crowned. Historically the Swedish initially defeated all these nations with some major battles such as the battles of Kliszow, Narva, Fraustadt. They installed a king of their choosing on Poland-Lithuania, but ultimately got defeated, when they marched into russia (cold winters and a lack of supplies, funny how often that happens) at the battle of Poltava (1709). You then have a war between the ottomans (who were sheltering the Swedish king, Carolus Rex) and the Russians. the Swedish-Russian war still continued mainly in Finland, but with the loss of their main army at Poltava and the king exiled in the Ottoman empire the Swedish were being defeated. When the king returned from exile in 1714, Varoius other nations started declaring war on Sweden. Britain declared war on Sweden in 1717 after the Swedish king negociated with the Jacobites. After the death of the swedish king in 1718 whilst besieging the Norwegian fortress Fredricksten, the Swedish started to negociate peace with other nations, finally coming to peace in 1721. The war ended with Sweden no longer a major power, and Russia becoming one of the greatest powers in europe.
I think it would be an interesting campaign to play. Could you, as Sweden, invade Russia Successfully? As Poland-Lithuania, could you prevent the dethroning of King Augustus II? As Russia could you finish the War early, defeating them at Narva? As Sweden, could you get more support from the Cossacks, rather than the 3000 supplied by Ivan Mazepa, which could potentially turn the odds in your favour for the battle of Poltava?
Anyone who listens to Sabaton should be familiar with this war, on their album 'Carolus Rex', the songs 'Carolus Rex', 'Killing Ground', 'Poltava', 'Long Live the King', and 'Ruina Imperii' are all about the events of the great northern war. They also sing about the 30 years war, which also involved the Swedish empire. This may also be an interesting era to play, maybe more so than the great northern war, but I have more interest in the great northern war personally. (I like the songs more)