King Frederick the Great is old and dying and so is the Prussian army. An army said to possess a state is in need of reform. Will Prussia live on its past glory and watch as the Habsburgs take a firmer grip on the Holy Roman Empire and expand to the west, or will it, perhaps, gamble for supremacy and unite the small German states under its leadership? You already have the support of some of them, such as Saxony, Hesse-Kassel and Hanover. The recent War of the Bavarian Succession was just a taste of Joseph II's ambitious plans.
The alliance with Russia is severely weakened by the Austro-Russian alliance made two years before. Catherine the Great obviously has other plans and is, together with Joseph II, considering wiping out the Ottoman Empire from the map, so an alliance with the Sultan would prove wise in order to keep the balance of power. Britain is currently without any major allies and is seeking new ones, threat from the expanding Russian Empire is a common problem. Also, be sure to protect Prussian interests in the collapsing Polish Commonwealth, otherwise Russia might take everything for themselves.
Due to Prussia's geopolitical position, Frederick maintained throughout his whole reign a policy of investing everything in the army and leaving very little for the navy explaining it by saying: "It is better to have the best army in Europe, than the worst navy in the Baltic."; so you will have to rely on friendly relations and alliances with other naval powers, namely Great Britain, the Dutch Republic and the Kingdom of Denmark, to ensure yourself from an attack by sea. Russia and Sweden are on the brink of war so use this to your advantage or even join Sweden against Russia.
Europe is shaken by the recent revolution in the American colonies, the people are on their feet, powers are shifting and new alliances are being formed. It is inevitable Prussia will be involved, in one way or another.