Allagator snapping turtle. To bad they get huge. But they do find the in alligaor ponds. You should put one huge pond in the eclosure. Or maybe even upgrade later. They do grow really slow. I know it is probably imptractical. I just think there cool.
id get a turtle with semi-aggression, i've seen caimen eat turtles so maybe a large growing aggressive turtle in my opinion is what i would do. Mata Mata or maybe even a tropical snapper or something.