I beg to differ. A venetian-wank is not impossible, only harder to realize, since Venice lacked the sheer amount of resources of other countries. Said this, there are some possibilities to explore. A few suggetions:
First: In order to keep her "colonial empire" in the mediterrean, Venice has to strike a deal with the ottomans. Without the sublime gate support, nothing can be obtained, or kept, in the eastern mediterrean. An alliance/vassalage with the ottoman empire would grant to Venice her status as main market for eastern goods. While the importance of mediterrean as commercial way declined after the discovery of Americas, it remained none the less a good avenue for any kind of wares. Moreover an alliance with the ottomans would probably save most of the venetian possessions in the east. Venice could even expand her empire conquering part of Dalmatia, Montenegro and Bosnia. With the riches gained from commerce, the manpower from Greece and Slavic possessions, the ottoman shield and a lot of shwred diplomacy, Venice should be able to keep at bay the other european nations.
All of this will need a lot of fine tuning, given the ottoman thrust toward Ungary and Austria, so maybe you'll want an ottoman empire more focused against Egypt and Persia (your man here is Abdul Hadi Pasha, I don't know enough about the matter).
Second: During the second half of the 16th century, Venice were pressured by the Pope to enforce strictier laws against eretics. Even if initially Venice resisted, in the following years the Doges had to cave in. In TTL this must be avoided. Venice have to stay a free place for everybody. This would lead to all the greater minds of the 16th and 17th century to seek shelter in Venice, making so the Serenissima republic a lighthouse for science and knowledge (Tip: a new universal library like the Alexandria'one, would be nice).
Third: In the 18th century the power of the ottomans will start to wane, so you'll need a new protector: the british fit the bill. Have the venetians strike an alliance with them. Backed by the british, venetia could begin to unify the peninsula, starting maybe with Naples and Sicily...
If eveything plays out nicely (and since it's a wank, why not) Venice could be master of a lot of Italy by the end of the 18th century. After the napoleonic storm, Venice could start to colonize Africa and maybe finance the Suez channel together with their old buddies, the ottomans.
To have colonies in america, Venice, or for the matter any other mediterrean nation, must have good relations with Spain, since they can cut off the access to atlantic quite easily. This, of course, goes against eveything I've written before. Even with Spain blessing, though, Venice will have to confine herself to the caribbean island ignored by the spanish. I would hearthly reccomand Barbados island, easy to defend, good for sugar cane and, simply put, beautyful
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