If you are just looking for a general scenario, perhaps a POD where the War of Spanish Succession never happens. The Austrians, lead by Prince Eugen, have more resources and are able to really crush the Ottomans in the early 18th century.
The Habsburgs are able to sweep into the Balkans. Wallachia defects. Then either in this war or the next one, the Bulgarians rebel allowing the Austrians to take even more land and/or establish a tributary state. A third war finally finishes off the Ottomans and pushes them out of Europe entirely.
This would generate a lot of butterflies, so I don't know how it would impact the rest of the setting in your Vicky II game. However, it seems like you just want an easy explanation to justify the challenge.