I see the first most likely but i prefer the second because it would be cool if animals that are not safe for a petting zoo also interact with guests through a glass window or something.
I could imagine new barrier attachment (like the walkthrough gate or dome enterance).
Interactive barrier - railing
Certain animals (llamas, fallow deer, hypothetical goats) would come to the railing and wait for head pats.
Interactive barrier - glass
More dangerous animals (big cats, apes) would come to the glass and paw at it.