The siege of Malta in 1565 between the Ottoman Empire and the Knights of Malta is probably one of the most famous battles in history, and one of the most celebrated early European victories over the Ottomans. It was also quite contested throughout.
What if the Turks had won through and forced the Knights off the island (or slaughtered them, as the case may be)?
Well, it might scare the Spanish and Italians a bit. It would also free up a burden on Ottoman shipping.
This might alos result in a earlier equivalent of the battle of Lepanto, and if the aim is to achieve an Ottoman-wank, a victory. Which would be more likely then OTL battle of Lepanto in any case, as it occured at a relatively innoportune time in naval terms for the Ottomans. Perhaps the early aim of the Ottoman campaigns would be Sicily, Tunisia and Southern Italy, rather then the Venetian possesions. In which case the Ottomans may be able to face off against Spain and Venice seperately, although Venice remaining neutral is unlikely.
OTOH, it might serve to unite more of Europe against the Ottomans...so an earlier Great Turkish War, and accompanying decline perhaps.....