What's the best nation to play has? It looks to me like Brandenburg, since they have quite a few militaristic ideas to start with.
Still there are some tricks, 1) annexing/releasing 2)convert to protestant/reformed and converting othersCurrently none as Imperial Authority is broken (at least on all of my 1.4.1.1 games). When defending someone from an outside nation you get 5 IA from responding to the call and zero for winning the war. The severe lack of IA means you'll never be able to turn into one nation.
My favorite is still Poland. If you suck up to Austria from the start, you should be able to get your capitol into the HRE early. Take down the orders and cripple Russia first, then focus on the HRE, taking 3 elector vassals to assure you the vote. Keep conquering in all other directions, and when the IA situation gets dicey: start adding all the provinces you grabbed into the HRE, gaining you huge amounts of IA in the process. When i did it i was able to enact 3 reforms in 1 day.