Fruzhin Shishman would make a good candidate, and was alive at least until Varna, and likely longer.
Presumably Venice would be playing godfather to the crusader states, offering protection for trade privileges and control of strategic outposts while being generally meddlesome and turning a profit. They could potentially have a similar but more limited role in relation to the Turkish successor states, but even with their history of siding with profit over faith the Venetians might see Turkish enclaves in Europe as threatening and help the Balkan Christians to strangle them.
There are other candidates as, interestingly, the family which ruled Bulgaria during World War I descends from the rulers of the Second Bulgarian state.