Started a new game as Brandenburg, after a couple fails as Poland (picked the wrong idea) and Milan (got owned, chain wars with no manpower against lucky AI's god-generals).
Opening moves: Made claims on Altmark and Vorpommeren, alliances with Bohemia, Saxony and Poland. Warred TO for my Neumark core with all 3 allies, took it + Danzig. Then Poland left alliance due to rivalry with Bohemia. Next I took Vorpommeren from Pommerania, who was also getting warred by TO. (Saxony had taken Altmark on it's own and I was growing nicely - I wanted to keep them as an ally for longer, but soon they ditched me.)
Bremen had expanded into 2 provs and given me the Dishonourable Scum CB, so I used it as a pretext to vassalize them. Vassals are a great way to grow - they give you some money, and raise more troops themselves to help you than you get in force limits for owning the land directly.
Saxony starts with Thuringia in a PU, so after Hansa snatched Mecklenburg, the north was pretty consolidated. My Diplomat went to work making claims in TO's Pommerania, but aside from taking those 2 cities there's not much to do.
With Bremen vassalized, Pommerania is easily liberated:
Having less place to expand actually worked well for me, as it kept my AE down. The new plan:
-keep Bohemia happy and allied
-diplo-annex Bremen, then make claims on Hansa's port towns and take those
-keep an eye on TO, as I'll need Ostpreussen to form Prussia later. Vassalization and Conquest both options, but mainly just keep them away from Poland.
-look for other opportunities to diplo vassalize or force PUs
Speaking of Poland. They were overrun by Orthodox Zealots and forced to convert, but have embraced the new faith:
This would eventually contribute to the end of their PU with Lithuania, who has managed to stay catholic despite living in the land of the Orthodox. Catholicism has no bonuses really, so I'm thinking this is actually a great plan for Poland.
After a few minor wars helping Bohemia, the next major opportunity: Saxony's crown is contestable, and I have a RM from the good old days. Only issue is their long standing Austrian alliance. Luckily Austria is fighting Venice already, so I take the chance to strike, with Bohemia and Bremen helping.
Austria was clearly doing worse in the other war than I thought. An easy victory for some ducats, and most importantly, a huge union member, was won. With a dedicated diplomat there (Diplo Ideas too good), I was able to repair relations before my king died, meaning the PU would keep going without problems.
The next 20-odd years were quiet. I diplo annexed Bremen. Built up my Constables and Workshops. Got the mission to build up my fleet so did that too (Trade Fleets only ofc). Keeping relations high was the most important way to expand, so no aggressive wars. I did help fund rebels in old Prussia after Poland+Lith took the lands. Over 30k rebels spawned and kept Poland very beaten down, eventually flipping the provs back to TO. Also, my von Hohenzollern king died without heir, replaced by a Polish-descended Jagiellon (and that's when I noticed the Polish-Lithuanian union had died - 2 different rulers from the same dynasty as mine).
After Bremen was annexed I went ahead with the plan to make claims in Bremen proper and Hamburg. But uppity Denmark wanted Danzig and brought it's kids to the party. With high losses on both sides they were pushed back, and eventually I got Holstein freed, to restore Imperial Unity (there were fewer than 25 states left and I wanted the bonuses back, you see).
A few years later, Bohemia fell under a union with my completely by dumb luck. I now have a tripple union, just like the Scandinavians: Saxony and Bohemia under Brandenburg.
The Tripple Crown then went to work bashing the Hansa, taking Hamburg+Mecklenburg (for mission). TO is still free and is now a high priority to deal with.
Here's the world near the turn of the century:
The eastern Archbishoprics are all doing well (Trier just took some land from Munster, but I have a feeling it'll be back). Sadly they can't be diplo-vassalized at all due to no Royal Marriages allowed, so my future peaceful expansion opportunities are limited. The Scandinavian union has fallen apart, as has the Polish-Lithuanin ones.. all those countries have separate rulers. Venice and the Pope are doing well to the south. NFI what's happening in Iberia.