My opinion is simple: if a zoo can't reasonably be done in one day, it's too big.
You say that, but I would never say Tierpark Berlin, Zoo Berlin, Pairi Daiza or Zoo Prague were "too big" - and all of them are pushing the one-day mark to breaking point.....
That's sort of my original point thoughthe ideal for me would be that a zoo isn't pushing the one-day mark to breaking point, since I'd either have to a) accept it as a 2-day venture, or b) very diligently plan a route and keep to a strict schedule so that you can see everything in a day... neither of which sounds as good to me as visiting a zoo that takes most of the day, leaving some time for a meal break and lingering longer in some areas.
I personally am not the biggest fan of the idea to have to stay over at a hotel in a zoo. Because that would make the zoo trip end up being very expensive! Daytickets + Hotel + food ... and perhaps even a souvenir...
This all will definitely make a zoo less attractive for me to visit since you'll be paying a lot of money and require two days to visit to get the full experience.