Tasmanian Devils fall into the essential category for me. Not because I think of them as staple zoo animals (from a world wide perspective) but because even losing one single animal from Oceania would create a hole in the roster. Like South America, Oceania needs just about every animal it has without losing anything in the sequel.
For South America and Oceania it isn’t “one step forward, two steps back” it would be “one step forward, four steps back”.
I’ll admit, even if they aren’t the most charismatic, any South American or Oceanian animal falls into “essential” territory for me.