Crete, an island, should be easier to defend for Venice than the inland Morea, so there is more chance that Venice could it until Napoleon.
But, one thing that might be able to single-handedly turn around Venetian Republic is somehow gaining Lombardy, probably via trading away Dalmatia. Lombardy is very populous and wealthy. In addition, gaining a region as big as Lombardy/Milan, probably bigger than Venetia itself, on top of Greek Crete as mentioned above, would have likely forced the elite to make major concessions.
Both are difficult, as both require naval and land forces. Navy's of this era are heavily dependent on economic output and will only become more so: the traditional Venetian advantages of the Oak reserves of Dalmatia and mass production, the former paling in contrast to the forestry of the Baltic and India (yes India, where its cheaper to produce ships of the same quality after 5 decades of quick learning and a captured Portuguese ship or two) and the later hampered by the really conservative galleys-great technique means nothing producing older manpower intensive ships. Armies on the other hand are getting larger, more professional, and more expensive (gunpowder imports from India, warhorse imports from Russia, warhorse stables, large standing armies) none of which Venice wants to pay for or compete with. Crete isn't the local nexus of trade anymore, nor a stepping stone to the Spice trade anymore, nor one of the only two sugar producing areas in Europe anymore; just nothing worth it there outside of strategic location which requires power to leverage in the first place.
Here's an idea. Instead of trying to maintain its Balkan empire, it turns towards North Africa instead?
Could Venice take Libya, especially after Egypt betrays the Ottomans?
Sure it can take and hold it, until either Spain, England, Naples, France, Ottomans decides otherwise; even with a power backing it the question then becomes how long until that major changes its mind or trades Venice off? Taking more ground won't change the poor mobilization of resources outside of Venice nor would it make the economies, armies, and population of the surrounding kingdoms relatively smaller. Military expenditures in the economic hinterlands of the Atlantic economies won't solve Venice's economic and political issues; just depletes its resources in taking it and accelerates its problems when the more powerful neighbors push back.
Military opportunities
The best case would be a serious ass-kissing campaign, while taking the time to catch-up. This isn't the fall of Byzantium when colonies were up for grabs at fire-sale price and the major kingdoms lacked navies and taxes, just look around at the military expansion opportunities:
To the North are the poor Swiss and relatively well off Robo-Hungarians; both are militarily adept and the later with an immense quantitively advantage.
To the West is Milan, Savoy and France, both Italian states are peer powers negating easy victory and often backed/invaded by the French Giant that can and has smacked down the Venetians if they felt like it.
To the South is the Papal States-can't touch this without Spanish or French approval and Spanish Naples & Sicily which are less militarily adept but still dwarfs Venice in quantity and is experiencing growing prosperity and inequality in its empire.
To the East is the Ottomans, which unlike Venice was trying to reform and hold on to its territories against the Spanish and Austrians which it did for the most part with the exception of the Russians kicking their teeth in up north; even being forced on a defensive posture they are still capable and its a century too early for nationalism to seriously hurt them. Even if they took land what do they gain? The Balkans are full of mountain bandit and ethnic strife being the frontier of 3 faiths, Kosovo has great mineral wealth but why would the Ottomans give it up and without railroads its hard to exploit the inland mines, the North African coast was only an outlet for the desert caravans (who have their choice of entrepôts) whose main value has been bypassed by Atlantic trade, Egypt is its own can of worms with the Mamluks and Berber extortionists and with the rounding of the Cape two centuries ago less a monopoly (not counting the ancient Persian Gulf to Trebizond alternative).
None of these lands are easy and those that are plausible are poor, painting the map doesn't make Venice a stronger regional player; just leaves it stretched out and depleted.
As for population 18th century Venetian Republic was on the lighter side at 2 million (similar to Portugal or Netherlands), though effective mobilization is lower than that of say the Lower countries due to wealth inequality and dispersed and decentralized nature of territories. Problem is as it always is with the Venetian Republic is the lack of natural resources and defensible borders (outside the city), the best things going for it are good agricultural land, relatively okay position (at least during the 18th century) with the possibility of being in play between the Germans, French, and Spaniards.
The first thing is simple, Venice is no longer the economic nexus and its laws should reflect that if not for the self-interest and vanity of the Patricians; Verona on the other hand was a natural epicenter amidst Terrafirma and the sea making it a natural center of trade and light manufacturing. Second, they've gotta open up political and social power/advancement. If it takes several generations, lots of bribes, and Patrician sponsorship to get into the upper strata then some cleaver and ambitious colonials will be turned off or as we see simply settle for regional prominence. Similarly while the Patrician unity, handouts, and such are great in the short run they have to balance it with some pruning. Part of the reason the city became the gambling and whoring capital of Europe is because there were so many uninterested, subsidized, but not rich enough to get wed (weddings had turned into a giant dick-waving contest with massive dowries and pageantry by now) Patricians drinking, gambling's, and whoring; which by itself was a pleasant side effect but should be capitalized upon like a proper tourism industry instead like a dealer and not the druggie who deals and uses. more to come later.
