Personally, Knoxville and the Tennessee Aquarium are both on the most direct route from Detroit to Orlando and are great options. There is also a small zoo in Chattanooga that could be combined with a visit to the Aquarium.
Overall though, this really depends on how far out of your way you are willing to detour. Metro Richmond, for example, is 4 extra hours of driving according to Google Maps. While, it is a pretty decent privately owned zoo, I don't think it justifies that much extra driving. (EDIT: Metro Richmond is in Richmond, Virginia, which is along I-85, not I-75 btw, so not sure if it is the one you meant to put here as I see there is a Richmond, Kentucky and I don't think there is a zoo there.) Jacksonville would add an extra half hour or so, while Louisville adds about an hour and Mesker Park adds two and half. I visited both Mesker and Louisville on the same day years ago, but that might depend on seasonal hours in order to make it work (also note they are in different time zones as that almost got me).
I have a feeling you left these places off your list because you have already been to them, but another option along the most direct route would be the Cincinnati Zoo, which could potentially be combined with a visit to the Newport Aquarium right across the border in Kentucky. Could also could do Zoo Atlanta and Georgia Aquarium, as I've combined them into a single day before as well.
If you are willing to detour as far as Metro Richmond, it looks like the National Zoo is along that same route and I would recommend a visit there over Richmond. If you are willing to go as much as 4 hours out of your way, a lot of options open up to you. North Carolina Zoo is a good option and would add about an hour and 45 minutes to your trip. Nashville adds about 1.5 hours. Birmingham and Montgomery both add about 2.5 hours. There are some smaller places along your route as well, such as the Santa Fe Teaching Zoo in Gainesville, FL.
Of all these options, I'd probably tell you to stick to the most direct route as Tennessee is a top level aquarium (probably third on my list right now behind Shedd and Georgia, but I flip back and forth on it and Georgia) and the zoo in Chattanooga is a nice visit as well.