My initial goal as BB is Pomerania > TO land. Generally, ally Saxony, ally Mecklenberg, improve with/RM Poland so they want to ally later. Fab on Pomerania's capital. Attack Pomerania, call Meck for land. Since you fabbed on the capital, Meck should flip it to you if they get the siege leadership somehow. Then vassal Pom. Ideally you only have to give up one province to avoid a trust hit with Meck; they'll immediately return it when the Emperor demands.
If you have to, maybe take a Pomeranian province yourself so you can fab on TO; in my current game I got lucky in that Pomerania put some claims on TO at the start, so I just used those. Once you have a claim on TO, ally Poland, dec TO, call Poland for land. Get occupations on Danzig and Konigsburg; as these are the "key" lands you'll need to form Prussia. Assuming Poland has the PU on Lith, you can feed LO lands to Lith to satisfy the Poles. If you just take Danzig and Konigsburg in the first war, you can probably avoid the trust hit without giving the Poles any TO land.
Once all that is done, I tend to expand back west; basically, you're looking for any chance you can get to control trade centers in the Lubeck node. I usually vassal > integrate, as it keeps the AE lower and avoids the whole unlawful territory bit, but you can probably directly conquer if you want. Free cities may have to be left until later, unless the emperor is getting run over by Ottos or something, in in which case he may just decline the call. You'll have to eat the penalties for unlawful, though. When/if Sweden breaks free, or even before, you can go after Denmark for their capital, to get the trade power there. Other than that, wherever you see an opportunity to eat dev.
Idea groups - Either influence or diplo as a first group. I'd probably prefer diplo, as you get the + 1 diplomat and + 1 relations right up front, you can keep your diplomats busy improving relations to help avoid coalitions. Most of the annexations will be small, so influence probably isn't as helpful here, though the additional dip slot and extra AE reduction would certainly be nice. Taking both is certainly viable, but I don't think necessary. For admin groups, I like Econ, as you get some nice policies with Econ + Off and Econ + Qual; Innovative gives you another couple nice policies; if you don't bother with either humanist/religious you can definitely take it in that slot, otherwise you're starting to run out of idea picks. For mil groups, I'd pick Def first (probably second overall) and the qual/off in whichever order you prefer. You could maybe get by without off if you wanted, but why? If you're doing limited expansion, quan probably isn't needed, but you could pick it up somewhere in there if you have the idea picks. More manpower certainly never hurts, especially if you have the opportunity to drill between wars.
