The best way I think it could happen is if the Ottomans are annihilated by Timur in 1402 leaving no Ottoman invasion of Europe. This means Venice, far stronger than Byzantium, can move into the Balkans.
However, the main problem is that Venice doesn't have much inland power projection. She'd ned either an ally or a mercenery force to conquer places like Slovenia.
Venice's maritime empire collapsed because of Ottoman aggression and Portuguese competition in the spice trade (with her Indies colonies). With no Ottomans she can hold onto the E. Mediterranean and those colonies are far richer than Dalmatia. Also, even if she did pursue an expansionist policy (as she did in OTL) it would be in Italy, which is far richer, culturally closer and far closer than the Balkans.
If, however, she desperately needed woo (for a large fleet, maybe for defence or something) then she could be persuaded to fortify Istria and the large forests there. Having a foothold inland would mean she could expand north.
If she conquered Ragusa then she could bring parts of Albania into her empire, however Ragusa had a strong town identity and probably wouldn't go down without a fight.
Also, we have Serbia to contend wirth. Under her legendary king (I can't remember his name sorry Serbs but I acknowledge his awesomeness) they almost conquered Constantinople. If Venice began nibbling away at her rear (don't be so immature) they'd be sure to put a stop to it (no means no).
The fact is is that Venice was a mercantile society- any venture had to have a potential profit (consider the 4th crusade etc.) and taking windswept mountains has no profit incentive.
But it would be fun, and what else are we alternate historiographers but tinkerers in time nad space, seeking to remake history under our own auspices. We're like generals, statesmen and Emperors who can't be bothered and have internet access.