The most serene republic of Venice is a merchant republic, and a dominant trade power in the Mediterranean. As a merchant republic, it gets one more merchant compared to other nations however it faces issues with reduced republican tradition if it expands to own too many provinces. For this reason, having a vassal to feed low value provinces may be helpful. With Mare Nostrum, Venice can also seek protection by forming a trade league with nearby OPMs, and create them (as trade cities) from its conquests.
Historically the republic grew rich from trade with the bordering empires. The trade with Византия was fruitful, but as any savvy businessman the Ventians looked to the new holders of Constantinople after its fall to the Ottoman-Turks. The Османы are the major power of the region, but with clever diplomacy and the riches of Mediterranean trade the Venetians will prosper.
Venice itself is in good position to expand in many directions - to the minor nations in the Balkans, the Eastern Mediterranean and the Italian nations. The last one, however, means meddling with the Holy Roman Emperor (HRE), usually Austria, which is a more powerful military nation than Venice, at least until 1490, when the Shadow Kingdom event will cause the Italian HRE princes to leave the HRE.