Historically, the only way Venice could have been captured was by blockading it (there were no city walls and the islands were heavily built on with no way anyone could land their soliders) and thus starving it into submission, which is what the Genoese tried to do in the 13th century. Therefore, any interesting feature would be to make the building of city walls in Venice impossible, have the city stand on only one square of land (like those cities in Civilization which could only be taken with Marines and such) and thus have Venice fall after blockading for x turns.