The obvious space to build a new exhibit in my opinion is the far-too-large Camel exhibit. You could build a small but nice exhibit complex on that site. You could also build an animal house near the Reptile House if you need too.
There's no need for the zoo to construct new buildings on top of other exhibits if they wanted to develop things like a small mammal house or invertebrate house. They already have numerous underutilized indoor spaces. The Wildlife Interpretive Gallery could easily house more live animals in place of things like their Science on a Sphere gallery. Invertebrate-wise, that building already has live butterflies and a Partula interpretive display (no live snails). There's also the old penguinarium that will be available for redevolopment once the repairs to Polk Penguin Center are completed, the Ford Education Center that houses several non-animal attractions which I don't think get much visitor use (4D theater, simulator ride, photo gallery), and some indoor holding like the old hippo barn that were publicly accessible once upon a time. However, the reality is that Detroit Zoo's emphasis on animal welfare has seen it move away from attempting to display a large collection. The Polk Penguin Center epitomizes this, as a $32 million major exhibit development consisting of exactly one enclosure. I'll be happy if they attempt a conversion of the old penguinarium, but anything more than that is probably wishful thinking.