A couple of options for animals I'd like to see in a walkthrough exhibit:
Tree shrews - One of my favourite mammals - almost a proto-primate - but I will accept they are just too small to be habitat animals. I would personally choose the Northern tree shrew, as they are the species most commonly kept in captivity.
Chameleons - While the standard exhibit is perfectly fine size-wise for a chameleon, I just worry that putting them in the box will mean that many of the chameleon's unique behaviours will be lost in the process. If I had a choice between a chameleon that didn't climb or feed and just sat stationary or not getting a chameleon at all, it would personally be a very difficult choice. I would get a large species, like the Parson's chameleon, and have them in the walkthrough exhibit. They are as active, if not more so, than the sloths and are as big as cats. Get one to occasionally fire its tongue off into a bush and come back with an insect, and that would be perfection.
Chameleons can definitely live in walkthrough enclosures - the desert house at Nuremberg Zoo has free-roaming Mediterranean chameleons, and this picture shows a panther chameleon in Zurich Zoo's Masoala Hall.