Being satisfied with the North Sea Empire, I've started a new game returning to the Teutonic Order, but unlike last time when I joined the HRE for protections from the Poles only to then leave the HRE to form a Prussian Empire eastward this time I again formed Prussia but am taking the HRE diplomacy route aka "aggressive negotiations".
Unlike the traditional pure Germany conquest path, this path has you take more of part in the diplomacy of the HRE, though I've also had my share of wars to make the borders look more Prussian (Silesia, Brandenburg, Pomerania).
Particular to the diplomacy missions, I subjugated Brandenburg so as to have control of an elector as well as took Diplomatic Ideas for my third idea slot to get the relations and diplomatic reputation for another mission.
Next I have to become an elector myself but that won't happen for a while because it's the age of reformation and I went protestant and Austria, the Emperor, doesn't like me because of that, so I now have to wait until I can lead the Protestant League and start the League War (30 Years War basically) against the Imperial Catholics, which will mean a clash of the titans between Prussia and Austria (you're welcome
@kwisatz ). I'll then need to win the war either by Peace of Westphalia or by making Protestant the dominant faith.
From there I should more easily become an elector as well unlock the mission to secularize from a bishopric into a proper Kingdom of Prussia, and then diplomacy my way to becoming the Emperor myself, then take the decentralized mess that is the HRE and go down the path of centralization reforms.