So we need a Cossack-wank. A way to keep the Hetmanate independent of both Poland and Muscovy, and also to paralyze Moscow and keep it from dominating the Cossacks.
To start with, this would not wank the Hetmanate above its neighbors and the fact remains that on its own the Hetmanate was not sustainable against its neighbors. It did not have "natural borders" and was too weak militarily. This is why the successive Hetmans had been looking for the Polish, Muscovite or Ottoman protection.
Then, when you are talking about the "Cossacks", you are actually talking only about the Ukrainian Cossacks because those of Don already had been Russinan clients and had little to do with the Ukrainian ones.
You also want to break the power of the Crimean Khanate, to stop the bleeding of manpower over through slave raids.
This could be done either by Russia (as in OTL) or by PLC (if it is much more powerful than in OTL) and prerequisite would be a control over Ukraine (the most practical way to get to the Crimea). So how are you going to reconcile this with wnaking of the Hetmanate?
A tall order to have all of them happen in quick succession. I might suggest that perhaps having Sweden turn its attention east and smash both the PLC and Muscovy would help—Gustavus Adolphus tries to pull off a vision of uniting all the countries bordering the Baltic under his rule.
He did not have an army powerful enough to even defeat the PLC in the field or to conquer Danzig. For his purposes it was enough to get possession of Polish Livonia and Russian Ingria. During the Time of Trouble the Swedes occupied Novgorod for a while but later gave it back. Not too mention that GA was a little bit dead well before the Hetmanate was created.
A combined Time of Troubles and Deluge that leaves everything from the Urals to Silesia in flames. The Cossacks take the opportunity to carve out a Ukrainian state and, with Swedish help perhaps, win some decisive victories over the Tatars (or maybe become vassals to the Ottoman Sultan and sell Poles into slavery?).
Leaving aside not very convincing doom and gloom scenario, success of Khmelnitsky uprising was mostly due to an alliance with the Crimea. When Khan decided that alliance with the Poles is more beneficial, Khmelnitsky's successes ended forcing him to look for another protector. Actually, choosing Sultan was one of the options but it did not get enough traction (which did not prevent future alliances with the Ottomans).