Well, I would recommend playing as Brandenburg, Austria or Bavaria if you're looking for a Germany game. Basically, instead of conquest, your main means of expansion is PUs and vassals. If you already know all about PUs, get stuck in. If not, there's loads of threads about them. Bavaria and Austria are both very strong countries, and Brandenburg has the benefits of holding some of the required cores, and being very close to the rest of them.
In order to become Emperor, you must be a monarchy, not the lesser in a union, or a vassal, and your ruler must be male. Until you've passed loads of reforms, you need to secure votes from the electors. You do this by vassalising them. If you annex them, then the electorship will pass on to some other country, and you won't be any better off. If you vassalise a majority of electors, you're pretty much always going to be emperor. Make sure they are the same religion as you, and if you can get a conversion of you and the electors to another religion (probably protestant) your position will be very safe.
Once you're emperor, you can defend the empire, release states, and convert states to your religion to gain influence, and you can use this to pass reforms. If you don't want to form the HRE, don't pass the last reform.
You should use your position to gain PUs and inherit the required countries. Remember that once you're emperor, you can't get the unlawful province event, so it's only your infamy burn that holds you from taking provinces of theocracies and the like, which you cannot PU.