changing the singular experience that is Byzantium: just try to hang on to as much as possible towards a "conquer Europe" seems less tempting. And at least currently Venice's specialty is being advanced, rich and influential despite having only very few cities (also: their stability is really bad), conquering 10, 15 cities with them just seems...meh.
I'm not against changing Byzantiums UHVs and not even against adding a "conquer XYZ"-style UHV but in my opinion it should be one that is best attempted after the Mongol invasion not one where you have to focus on expansion right from the start.
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The problem with the 'singular experience' of Byzantium and Venice is that they are both pretty boring. After about 1000AD, Byzantium just becomes "click end turn, kill Selkjus / Mongols, click end turn, switch gold to 100%, win". Similarly with Venice - you get the first UHV almost by default with your starting settlers and Ragusa usually flipping, the Constantinople UHV just requires you to take over a crusade a few turns before 1204, and the colony goal honestly just seems a bit silly. Defeating the Ottomans, who were the primary cause of the fall of the Venetian trading empire, would be a much more effective and challenging goal.
It's not like the proposed goal is to conquer half the world, either. Venice gets five or six ports as part of its current 1st and 2nd UHVs, and historically Venice was rich because it controlled so many eastern Med ports. The stability issues can easily be fixed by adding Crete, Cyprus, Rhodes, Crimea and Morea as historical territory.