Well if you want to make a mod with a focus on Venice there's in my opinion two options regarding the map:
#1: Only northern Italy with a direct focus on the city of Venice, regardless of most/all of it's territories on the dalmatian coast.
Possible map:
#2: Whole south-eastern europe, featuring Venice's biggest enemies: the Byzantines, the Ottomans (later of course), the HRE/Austria and of course the other italian states, though this conflict will be in a smaller dimension than in the first scenario.
Possible map:
Of course the 2nd one is a lot more work, but I think it would also be a lot cooler and more popular.