IMHO the best early modern POD is the Peace of Prague (1635) This was the first chance for peace in the Thirty years war and the only peace advantageous for the emperor .
- Formal alliances of States of the Empire among themselves or with foreign powers were prohibited, which applied to both the Catholic League and the Protestant Heilbronn League.
- The armies of the various states were to be unified under the command of the Princes as generals of the Emperor, to establish an Imperial Army for the Holy Roman Empire as a whole, which would fight against invading troops.
- Amnesty was granted to those princes who had fought against Imperial troops, with the exception of the exiled descendants of the former "Winter King", Elector Palatine Frederick V.
If this peace is fully implemented (after a victory about both Sweden and France ending the war), the emperor gains back control about the outer politics of the Empire. Though the different territories still enjoy a large inner autonomy, wars between different Holy Roman states are now quite impossible, unless the emperor wage one against a stubborn prince, reluctant to accept the authority and supremacy of the elected Emperor.
After that, I can see:
30 May 1635: Peace of Prague
Second half of 1635: Sweden, shattered by financial problems and the death of both Gustav Adolf and Axel Oxenstierna (ITTL), quits the war.
1636-37: France can't stand against the united armies of the united German states allied with Spain and a peace treaty is agreed upon.
1637: Moderate Ferdinand III becomes emperor.
1640/41: Execution Diet of Nuremberg: Peace of Prague is declared to be a fundamental law of the Empire. Also, united imperial military and foreign-policy institutions are created (slowly evolve into a modern government).
~ 1680/90: Mutual (succesful) defense against both France and the Ottoman Empire contribute to the unity of the Empire and consolidate imperial control over the different armies. Territories of princes allied with France are annexed to the Habsburg allodium.
1712: After long wars against the HRE, Spain and Great Britain, the United Provinces capitulate. They are given religious autonomy and a representation in the Imperial Diet, but their remaining colonies become property of the Empire.
Until 1760: The wars of the 18th century burden the budget of the imperial government. As a reaction, the Emperor decides to increase the contributions to be payed by the different states. This corresponds to a general centralization of the empire, e. g. in criminal and civil legislation or in weights and measures.
Many princes refuse to comply with the new restrictions, but they stand no chance against the imperial troops. More and more territories are subordinated to Major-Generals appointed by the emperor, while the local rulers loose most of their powers.
1763: The Imperial Diet assembles for the last time and formally accepts hereditary succession. After this decision, this feudal assembly dissolves forever.
Second half of the 18th century: the bourgeoisie, on which is bases imperial central power and which is the most important economic factor in the Empire claims for more rights in politics. The German enlightenment follows.
1789

: To reinforce centralization, the Emperor decide to dissolve the feudal State diets. However, the representatives of the Third Estate refuse to follow these orders. The army sympathizes with the revolutionaries and changes sides. A new Imperial diet (a true national parliament consisting of elected deputies from the different states) assembles in Vienna, which becomes the new German capital, and elaborates a modern and liberal constitution.
After 1789: The Holy Roman Empire sill exists as a German federal state with Italian, Polish, Hungarian, French, Dutch and Czech minorites enjoying broad autonomy.
Sorry for the long post.