The need to relocate the zoo from Kalemegdan is, as you said, self-evident. The problem is how to do it. Now nobody in Serbia has the expertise to organize relocation, even the (former) mayor Šapić admits that. The right way to do it is to hire some professional organization/company from the beginning, make several proposals for the new location, look at the pros and cons of each location, size, accessibility, other factors... I know it might take some time to make a decision, but moving a zoo is not something you do every year. For example, Ada Ciganlija has more open space. There is a golf course near the proposed location. Including that area the zoo could be close to 30 hectares - maybe I'm being "greedy" but I really think 14 hectares is small for a "new" zoo in a city of (officially) 1.6 million.
All that, the (former) mayor's proposal (to me) looks like a classic pre-election promise made for publicity. No other institution at the city or republic level commented or had anything to say about the proposal. Nothing has been heard about the proposal since the election. The zoo is renovating the old enclosures (elephants) and building new ones – albeit small in size – so no one seems to have told them anything.
I would like to see the Belgrade Zoo in a new location in my lifetime , but it should be really better, not some hasty solution.