I had a great Bavaria game not long ago - I only stopped since I'm a restart-aholic. Some notes:
1. For all my early conquests, I force vassalized and then annexed the targets. I took diplomacy for the extra diplomat and +2 relations.
2. Ansbach! I just get annoyed when this dinky province gets allied to a power like Austria. I day one took the mission to conquer them and declared one them before hitting unpause. This "should" get you into a 1-1 war with them.
3. Unite the Bavarian Lands! You are surrounded by rich OPM's. Declare war and force vassalize them. BEWARE, I actually had a coalition form against me which I find to be ultra-rare in 1.4 as I force-vassalized Ulm/Augsburg/Ansbach right away. Take your time to let your AE go down, and do a bit of improving relations. After these, Wurtemburg, and I believe it is Wurzburg for Nuremburg are rich Bavarian targets.
4. Take Oberpfalz and vassalize the Palatinate. You obviously need to take Oberpfalz to make pretty borders and it is Bavarian! Keep the Palatinate as a vassal. They are your dynasty, and an elector. After a while, if you inherit their lands you will inherit their vote too!
5. From here, you are in a good position to become emperor - there are a bunch of tiny electors on your doorstep, or do whatever you like!