Lion of the North

This timeline revolves around the king of Sweden, Gustav Adolf II (Gustavus Adolfus is what he's commonly called outside of Scandinavia) not dying in a calvary charge at Lutzen (1632). It also has minor changes to OTL (Bernard of Saxe-Wiemar dies 2 years early, Tortenssen isn't delayed by Heanistic League traders when he take the army across the Baltic, etc).

The Landing at Rostock (1625-1626)
Under orders from Gustav, Tortenssen takes the entire standing army into Rostock in 1625, in order to put Germany firmly into the Protestant sphere of influence. (this happened OTL in 1630, because of resistance by traders). This force was about 30,000 men, most of them ethnic Swedes and Finns. They were very different from the Catholic armies for a number of reasons. Unlike the Imperials, it was completely religously homogeneous, being 60/40 Lutheran/Calvinist. It was also ethnicly similar, all of them hailing from the Swedish Empire. Thirdly, it was one of the first standing armies, making it infinitely better than the predominately mercenary Catholic armies (I'm going to refer to the Catholics as imperials, or as Hapsburgs. Just know when I say Swedish Empire it's different from the Imperials.). Lastly, they had simply amazing artillery. Their gunnery and cannons were average, but Gustav had standardized the size and wieght of the ammunition and guns. This simplified the supply train consiterably, as the Imperials had to have ammunition crafted for individual cannons.

The standing force was intended only as a core of the army, as German mercenaries would be in great supply in Germany. After borrowing money from the burghers of Rostock, Tortenssen managed to hire 20,000 troops. With these, he marched straight up to the Duke of Pomerania, forcing him to sign the Union of Rostock. This gave Sweden complete control over Pomerania, and a useful supply base. (OTL, the Swede's supply train stretched all the way back to Stockholm). Tortenssen sent Horn with 4,000 Finnish calvary and several batteries of artillery to make similar deals with Wismar, Mecklenburg, Hamburg, and Bremen. Supported by the navy, these cities capitulated within a year. This gave Gustav control over the entire northern coast of Germany.

Brandenburg (1626)
Brandenburg had long been a bastion of Calvinism in Germany. It had also long been a bastion to the 7,000 Imperial mercenaries under the Count of Tilly (Yes, it's a real name). As soon as Tortenssen had landed, he sent troops into the surrounding countryside to keep the Imperials from foraging. As Horn returned from Bremen with 19,000 German Calvinist volunteers, he lay seige to the city. The mercenaries capitulated in 2 weeks due to the strong resistance the Protestant citykolk were giving them.